Lee Hart with our
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Lee Hart's LeCar EV

Electrifying Vehicles

EV Engineering with Lee Hart

"To invent, you need a good imagination and a pile of junk."
Thomas A. Edison

Hi! I'm Lee Hart... one of the founders of the Sunrise EV2 Project, and the chief financier and cat herder at the moment. I've been messing about with EVs since the 1970s (and even earlier if you count electric trains and sidewalk scooters). I've built many EVs myself, helped hundreds of people build theirs, and posted thousands of comments on the various EV discussion lists. If you hunt around the internet for EV information, you're sure to run across me! Here are just a few links to my work:

I also do this web site. But as you can tell, I'm no web designer. I could spend hours writing and polishing elaborate web pages for you; but they would still look amateurish. I guess I'd rather be working on EVs than on this web site!

So, in order to help YOU work on your EV projects, here is a "data dump" of condensed EV circuits and specs I've collected over the years. I call it...


Electric Vehicle Engineering
Keep It Simple, Stupid!

#1 -- EV Circuits: The Batt-Bridge Battery Out-Of-Balance Alarm

Batt-Bridge circuit

An EV's pack consists of many cells or batteries. In theory, they are all identical. In practice, they aren't. There will always be a "weak link" somewhere in the pack. That's the cell that limits your range, and limits how much you can charge before damage begins.

But it is difficult to know if you have a weak cell. Total pack voltage won't tell you until too late. The amount of circuitry needed to monitor every single cell can get very complicated and expensive!

The Batt-Bridge is a quick-n-dirty "idiot light" to give you a good/bad warning when any cell in the pack goes undervoltage (dead) or overvoltage (overcharged). It works with all types of batteries; lead-acid, lithium, nicad, or nimh. If it lights up red, back off on the current until it goes out. If it stays on even at zero current, stop driving or charging until you find the problem and fix it!


How it works: The Batt-Bridge divides the pack in half, and compares the voltage of each half. When the two halves are equal (within 1 volt or less), a green LED is lit. If a cell goes dead or begins overcharging somewhere in the pack, its voltage typically changes by more than a volt. This imbalance lights one of two bright red LEDs to tell you which half of the pack is low.

D1 is a standard brightness green LED. D2 and D3 should be ultrabright red LEDs for best visibility. R1 and R2 should be identical resistors, chosen to provide about 10ma at your pack voltage. The current sets the sensitivity and brightness of the LEDs.

Construction: Mount the LEDs in a pilot light holder. Install it on the dashboard where the driver can easily see it, and where it won't be washed out by direct sunlight. Mount the resistors at the battery terminal ends of the wires, so they will limit the current in case of shorts.

Batt-Bridge parts kit -- $10. Includes LEDs, resistors, lamp holder, and detailed instructions. Specify pack voltage. $5 for US shipping and handling. For shipping outside the USA, email for details.

Batt-Bridge, Assembled -- $20. Assembled Batt-Bridge in panel mounted lampholder, with resistors and detailed instructions. Specify pack voltage. $5 for US shipping and handling. For shipping outside the USA, email for details.

Zener-Lamp Regulator

#2 -- EV Circuits: The Zener-Lamp Shunt Regulator for Lead-Acid Batteries

The Batt-Bridge tells you that some batteries in your pack are less charged than others. With flooded batteries you can overcharge to bring up the low batteries, and replace the water lost in the ones that were already full. But if you overcharge sealed batteries, it shortens their life.

The Zener-Lamp regulator is intended for sealed 12v lead-acid batteries (though it can be adapted for other types). It bypasses excess charging current on full batteries, so the weaker ones can finish charging without overcharging those that are already full. It's a budget regulator, so even cheapskates won't have an excuse to murder their batteries because a real BMS (Battery Management System) would cost them too much.

How it works: Two zener diodes set the voltage above which it bypasses current. 6.2v and 6.8v zeners will bypass above 13v (full charge for a 12v battery). A #PR2 or #43 2.5v 0.5a lamp limits the current, provides a visual indication when it is bypassing, and acts as a fuse in case of overvoltage, miswiring, or component failures. The 10 ohm resistor is a backup in case the lamp fails.

Zener-Lamp Regulator

Construction: The zeners get hot! Mount them in large copper ring terminals for #6 wire. Fill the body with a thermally conductive epoxy (like JB Weld) to transfer the heat to the battery terminals. Solder the lamp and resistor in parallel. Connect them to the other ends of the zeners with short wires. Cover the connections with heat shrink tubing and epoxy to waterproof them.

Use: Near the end of a charge cycle, the lamps will begin to light. Set up your charger to be at a low current (like 0.5 amp) at this time. At this current, each hour you charge puts an extra 0.5 amphours into the low batteries whose lamps aren't lit. The pack is balanced when all the lights are glowing to some degree. This may take many hours on an out-of-balance pack. Once balanced, subsequent charges should take under an hour to reach this state.

Zener-Lamp Regulator kit -- $5 each, plus $5 total for shipping.. For 12v lead-acid batteries unless you specify otherwise. Includes all parts, with ring terminals for 5/16" bolts, and detailed instructions. For shipping outside the USA, email for details.

Zener-Lamp Regulator, assembled -- $10 each, plus $5 total for shipping.. For 12v lead-acid batteries unless you specify otherwise. Assembled, with ring terminals for 5/16" bolts, and detailed instructions. For shipping outside the USA, email for details.

#3 -- EV Circuits: The Doubler 12v to 24v Contactor Driver

Contactors are expensive, and their coils use a lot of power. This can load down your 12v system, and the coils run hot. The Doubler cuts coil power by 4:1, by using less expensive contactors with 24v coils (common in industrial EVs and the surplus market). It doubles your 12v to 24v to pull in the contactor quickly. It then holds the contactor on with 12v, using only 1/4 the power. This lightens the load on your 12v system, and the coil runs cool for longer life. For example:

Doubler Contactor Driver
      Schematic Albright SW-200 contactor with 12v coil:
-- continuous duty coil:  8 ohm, 1.5 amp, 18 watt
-- intermittent duty coil: 4 ohm, 3.0 amp, 36 watt

Doubler with Albright SW-200 and 24v coil:
-- continuous duty coil:  32 ohm, 0.4 amp, 4.5 watt
-- intermittent duty coil: 15 ohm, 0.8 amp, 9.6 watt

How it works: The "Switch" is whatever your EV uses to control the contactor. It can be the ignition keyswitch, a relay, or the solid state output from a controller like the Zilla. When the switch is off, capacitor C1 charges through lamp I1 and Schottky diode D2. C1 quickly charges, and the current falls to zero. MOSFET Q1 is off because its gate is grounded by R1, the contactor coil, and D2.

When the switch closes, R1 turns Q1 on. This grounds the + end of C1, and lights indicator I1. Since C1 was charged to 12v, its - end is now at -12v. D2 blocks, so the contactor coil has +24v across it and pulls in. C1 quickly discharges, and the voltage across it falls to zero. The coil is then held on at 12v by current flowing through the switch and D2. Q1 stays on, so I1 stays lit.

When the switch turns off, the coil's inductive "kick" is clamped at -24v by zener D1. R1 turns Q1 off, and zener D3 protects the gate of Q1. C1 now recharges through I1 for the next switch turn-on.

Doubler Contactor
        Driver Photo Doubler in Box Photo Doubler on Contactor

Construction: A bare PC board, parts kit, and assembled units are available. The board has holes to mount on an Albright SW180, SW190, and SW200 series contactors (or equivalent), or can be mounted separately in a plastic enclosure.

Doubler, Bare Board -- $10. Includes PC board, detailed instructions, and list of parts needed. $5 for US shipping and handling. For shipping outside the USA, email for details.

Doubler, Complete Kit -- $19. Includes PC board, all parts, and instructions. Soldering is required. $5 for US shipping and handling. For shipping outside the USA, email for details.

Doubler Assembled -- $29. A fully assembled and tested Doubler, complete with detailed instructions. $5 for US shipping and handling. For shipping outside the USA, email for details.

Doubler Case -- $5. If ordered with an assembled Doubler, it will be mounted and conformally coated at no extra charge. $5 for US shipping and handling. For shipping outside the USA, email for details.

E-Meter-Companion-DC/DC #4 -- EV Circuits: The Companion Power and Data Isolator for the E-Meter and Link-10 Meters

The Cruising Equipment E-Meter and Heart Interface/Xantrex Link-10 are popular high-quality meters used in many EVs. They display the voltage, current, state of charge, amphours in/out, KWH, time, temperature, and other factors. They can also send this data serially to your computer for data logging and analysis. This information is very useful for monitoring the health of your batteries, and extending their life.

However, the E-meter and Link-10 were originally designed for grounded 12v or 24v batteries. As such, they have a few shortcomings when used in EVs:


Adding all of this externally can easily add $200 to the cost of the meter. The Companion is a simple circuit board that includes all these functions (prescaler, isolated power supply, and isolated data output) at a much lower price. It simplifies installation and eliminates wiring errors. It mounts on the back of the meter, without increasing the depth behind the panel or extra little boxes. Build it yourself from the schematic. Bare boards, kit versions, and assembled Companions are also available.

How it works: R1, R2, R3, C1, and D1 are the Prescaler. If the pack is connected to W1, R1 scales the meter to read 0-100v. If the pack is connected to W2, R1-R3 scale the meter to read 0-500v. The meter has a configuration option to show the correct voltage with the prescaler connected. C1 is a noise filter (EV packs can be very noisy). D1 protects the meter from reversed or excessive voltages.

U3, C3, C4, and F1 are the isolated power supply. The heart of it is the Powerex DC/DC converter. It has an 8-16vdc input, and a 24v 100ma output isolated to 3500v. The capacitors filter out noise. Fuse F1 protects against excessive voltage or reverse polarity inputs.

U1, U2, R4, R5, and C2 provide the optically isolated data output. Two optocouplers are used for a symmetric output with equal rise/fall times, to prevent distortion or errors. The resistors limit the LED current, and C1 speeds up the switching for sharp clean edges. The output of the optocouplers gets its negative supply from the serial data output of the PC (which is otherwise unused), and its positive supply from the 12v system that powers U3 and the meter. The data output thus has standard +/-12v RS-232 levels.

E-Meter/Link-10 Companion Schematic

Construction: Everything mounts on a 1.9" diameter round PC board. Headers J2 and J3 use wirewrap pins, so the long tails reach the meter's screw terminal strip. U3 is socketed, so it can be plugged in after the screws are tightened. RS-232 connector P1 is taken apart and shortened, to avoid adding depth. Two screws secure it to the meter for additional support. (Note: Even if you bought your E-Meter/Link-10 without the serial option, it will probably have everything for it installed except the RS-232 connector itself. When you order a Companion, this connector is included in case your meter doesn't have one.)

Use: A shielded well-insulated shunt cable connects J2 to the shunt. A prewired 6' cable is supplied to prevent miswiring. The two ring terminals at the end connect to the small screws on the shunt.

The red wire to W1 or W2 should be well insulated, as it connects to the pack. If this wire is long, install a fuse at the battery end in case the wire ever shorts to ground.

The cable plugged into J1 provides +12v power, ground, and serial data output. A 3' cable is provided, but any length can be used. Separating J1 from J2 and J3 and using different connectors makes it impossible to mix up high voltage with the low voltage grounded wiring.

J3 is only used for options, such as a remote temperature sensor or alarm outputs.

Companion, Bare Board -- $10. Includes PC board, parts list, schematic, and instructions. Soldering is required. $5 for US shipping and handling. For shipping outside the USA, email for details.

Companion, Complete Kit-- $50. Includes PC board, DC/DC converter, all parts, and instructions. Note: Assembly requires a fair amount of skill. $5 for US shipping and handling. For shipping outside the USA, email for details.

Companion, Assembled -- $75. An assembled and tested Companion board, with DC/DC converter, cables, D-connector for E-Meter/Link-10, and instructions. Does NOT include E-Meter/Link-10. $5 for US shipping and handling. For shipping outside the USA, email for details.

Potter and Brumfield T92S7D22-12

High Voltage DC Relays and Contactors

You may have noticed that when you open a circuit (with a switch, relay, contactor, connector, or whatever), you get a spark. A little arcing is inevitable. But if it is not limited, it will shorten the life of the switching device, or even destroy it and leave the load still powered!

Switches, relays and contactors have voltage and current ratings, either printed on them, or listed in their data sheets. They can be pretty confusing! For example, here are the ratings printed on a Potter and Brumfield T92S7D22-12 relay:

Even more ratings are provided on the data sheet. But you don't need to be a contact engineer to understand all of this. It's sufficient to learn the basics, so you can pick a suitable contact for what you want to switch.

UL, CSA, and VDE are safety regulatory agencies. These codes tell you that someone other than the manufacturer has tested this part, and certifies that the ratings are honest. To get agency ratings, the part has to be able to switch the specified loads for 100,000 cycles. If you don't see any agency markings, the manufacturer is free to make up anything he likes. You'll often find absurdly high ratings on parts with no agency testing or confirmation. For example, automotive grade relays have no agency listings, and can only switch their rated loads for 10,000 cycles (or less)!

PRD-7DH0-12 Contactor

Let's look at the AC ratings. The higher the voltage, the lower the current it can carry. The first two values assume a resistive load. HP is "horsepower", i.e. an inductive load. Each horsepower is about 1000 watts; so 1HP is 1000w / 120v = 8.3 amps, and 3HP is 3000w / 240v = 12.5 amps. Inductive loads arc a lot more, so the current rating is roughly half as much when switching an inductive load.

Now look at the DC rating. Once you have more than about 30 volts across a contact, it will arc. Thus this relay only has a DC contact rating of 28 vdc at 20 amps. So why is the AC voltage rating so much higher? It's because AC voltages automatically go through zero 120 times a second at 60 Hz (or 100 times a second at 50 Hz). This automatically extinguishes the arc, so it won't last any longer than 8-10 milliseconds.

But on high voltage DC, once an arc starts it WON'T STOP until the contact spacing is very large, the current is very low, or some other mechanism stops it. The arc lets current keep flowing to the load, and also quickly destroys the contact. This particular relay has no provisions for switching high voltage DC; thus the low DC voltage rating.

The voltage rating of contacts in series add, because they increase the total open contact spacing. This is a double-pole relay, so you can wire both 28vdc contacts in series to switch 56vdc. Likewise, you can use a relay with four 30vdc contacts to switch 4 x 30vdc = 120vdc. Just make sure that ALL the contacts open and close at once (i.e. they are all part of the same switch or relay).

Potter and Brumfield
KUEP-3D55-12 relay

It also pays to look at the data sheet. Some switches and relays have higher DC voltage ratings at reduced currents. Schrack relays (now owned by Tyco) often have this data. For example, the Schrack PT570012 (Digikey PB912-ND) is a 4PDT relay with four 6 amp 120vac or 30vdc contacts that the data sheet also rates at 300vdc at 1 amp.

You can also get relays and contactors specially designed to switch high voltage DC. Several methods are used. First, much larger spacing between the open contacts. Second, putting more than one contact in series. Third, blowout magnets.

The Potter and Brumfield PRD-series is a common example. It is often used to switch EV chargers, heaters, and other DC loads up to 20 amps. It has two contacts, each rated at 125vdc that can be used in series to switch 250vdc. To get this rating, it has a blowout magnet, and extra-large contact spacings (see photo). It is available from multiple sources (Tyco, Deltrol, Magnecraft, etc.). The AC versions are far more common, so be sure to get one with the blowout magnet (such as the PRD-7DH0-12).

The smaller Potter and Brumfield KUEP-series is similar, but rated for 10 amps at 150vdc. It is useful for DC/DC converters and other smaller loads. It also has a blowout magnet (see photo), and two contacts pre-wired in series. Again, AC versions are much more common, so look for one with the magnet, like the KUEP-3D55-12.

I normally have these relays in stock for EV projects. If you need one, email me for details.

Electric Vehicle Battery Heaters

Battery Heating Blanket

No matter what kind of batteries you use, they will perform a lot worse when cold (just like people)! If you install about an inch of styrafoam insulation around your batteries, their own waste heat from daily driving and charging is usually enough to keep them warm. I put my batteries in such a box, with a removable lid. Leave the lid off in the summer, and put it on in the winter. A batt of fiberglass insulation is handy for insulating the lid, as it is nonconductive, won't trap vent gases, and molds itself around the terminals and wiring.

You'll need battery heaters if you don't drive every day, and live in a climate with weather below freezing. With 1" of insulation, you only need about 20-40 watts per square foot of battery area. With less insulation, more heat is needed. A typical battery heating blanket is shown at right. They come in various lengths and wattages, and sell in auto parts stores for $20-$60. It's a plastic bag, with about 1/4" of insulation inside and a long piece of nichrome resistance wire. The wire is attached to a thin sheet of aluminum foil, to hold it in position and (in theory) spread out the heat. The foil is connected to the AC line cord's ground wire in case something shorts.

You can use these as-is, but I've found they are a little too crude to work dependably. There is no thermostat, so the battery temperature is uncontrolled. There is no fuse, so if it gets wet or pinched it can even start a fire! For reliable operation, it is better to repackage them, and add a fuse and thermostat.

Battery Heating Blanket

Here's an inexpensive way to do it. Cut a sheet of aluminum about the size of the floor of your battery box. Take the heater apart, carefully separate the resistance wire, and temporarily tape it to the aluminum sheet. Space the wires out evenly -- if they cross or even get too close to each other, you'll get a "hot spot" that will fail. Connect the ground wire to the aluminum sheet.

Get a 10-ounce tube of high-temperature silicone sealant (intended for sealing furnace ducts and chimney flues) from your local lumber company. Apply it to the wires with a caulk gun. Cover it with aluminum foil or a polyethylene plastic sheet, so you can push the sealant around and squeeze out the air pockets without making a mess. If this heater is for flooded batteries, apply plastic to both sides with the silicone sealant to prevent corrosion and ground fault leakage currents.

Silicone needs exposure to air and moisture to cure, so it will take a long time to fully set. But with the plastic or foil cover, you don't have to wait before handling and installing it. The left photo shows one of my repackaged battery heaters.

Lay a sheet of styrafoam in the bottom of your battery box. Place the heater on top of it, with the wire side down. Now place the batteries on top of that. The wires will sink slightly into the styrafoam, and won't be pinched or damaged by the weight of the batteries. The aluminum will spread the heat evenly over all the batteries.

Batteries -- Lead-Acid

Let's anthromorphize a bit, and consider lead-acid batteries as alive; like the family dog.

Energizer Battery Dog Photo
  1. They need exercise; it's good for them. You get the longest life when they are worked to about 50% of their capacity at moderate loads. After they have been loafing for weeks, you will notice a distinct improvement just from giving them moderate exercise.
  2. But don't work 'em till they drop! If you drive an EV until it barely moves, the batteries are having a near-death experience! This is outright battricide, and a leading cause of early death.
  3. They need to be be fed regularly (charged). Feed as soon as possible after a workout; they don't like to sit around starving after use. Batteries left sitting for days in an undercharged state develop a condition called sulfation.
  4. Don't overfeed, or they get fat and have cumulative health problems and so die early. Chronic overcharging is a major cause of early death.
  5. Don't underfeed, or they can starve to death. Chronic underfeeding also leads to a weak sickly battery and an early death.
  6. Batteries can sit unused for months (hibernate) without needing to be fed. You don't need to put them on a trickle charger; just be sure to feed them occasionally so they stay near full charge.
  7. They need fresh, clean water occasionally. Sealed batteries have a built-in watering system, but flooded batteries do not. Be sure to check water levels, and fill with distilled water as needed (dirty water poisons them!)
  8. They need to be kept at reasonable temperatures, that you would find comfortable. Not too hot, and not too cold. Lead-acid batteries are "cold-blooded", so the lower the temperature, the slower they get. Likewise, they can't "sweat", so high temperatures cook 'em to death.
  9. Batteries can't talk. They won't whine when they're hungry, or cry when you hurt them. You have to check their state of health with instruments, like voltmeters ammeters and hydrometers.
  10. There are different "breeds" of batteries, each with its own good and bad points. Slow plodding workhorse floodeds, but long lived. Racehorse AGMs that are fast and powerful, but short lived. Using the wrong breed of battery for the application, or unrealistic expectations leads to disappointing results.
  11. And sometimes, it's just "the luck of the draw". For no obvious reason, identical batteries in the same vehicle will have some die young, and some seem to live forever.

The usual reason you see a used EV that says "needs batteries" is because the previous owner treated the batteries cruelly. Whether by ignorance or laziness, some or all of the above guidelines were violated. But batteries are replaceable, and it usually means you can get the EV "cheap".

But such problems can be cured. A little detective work to fix the problems, and then some tender loving care will go a long way toward getting the longest life possible on the next set of batteries.

Batteries -- Manufacturer Specifications

State of Charge (SOC)

Specific Gravity  Volts/Cell  6v Battery  8v Battery  12v Battery  State of Charge
     1.260        2.12-2.15v  6.3-6.45v   8.4-8.6v    12.6-12.9v   100% (full)
     1.220        2.06v       6.18v       8.25v       12.36v        75%
     1.180        2.03v       6.09v       8.12v       12.18v        50%
     1.140        1.99v       5.97v       7.96v       11.94v        25%
     1.100        1.97v       5.91v       7.88v       11.82v         0% (dead)
     0.030        0.017v      0.05v       0.067v      0.1v         max variation
   1. Measure after sitting without charging or discharging for at least 8 hours.
   2. AGM and starting battery voltages will be a little higher.
   3. Old battery voltages will be a little lower.

Cycle Life

                             typical lead-acid battery life cycles
Dept of Discharge(%)  ----flooded-----  ------gel-------  -------agm------
                       (Trojan T-105)   (Deka Dominator)  (Deka Intimidator)
        100            600 x 1.0 = 600   450 x 1.0 = 450   150 x 1.0 = 150
         80            800 x 0.8 = 640   600 x 0.8 = 480   200 x 0.8 = 160
         50           1500 x 0.5 = 750  1000 x 0.5 = 500   370 x 0.5 = 185
         25           2500 x .25 = 625  2100 x .25 = 525   925 x .25 = 231
         10           5500 x 0.1 = 550  5700 x  .1 = 570  3100 x 0.1 = 310
   1. Cycle life depends on type of battery, depth of discharge, and discharge current.
   2. The shallower the discharge, the greater the cycle life.
   3. Minimum cost per mile occurs when cycles x DOD% is at its maximum. 

The Sunrise EV2 Project, © 2007-2013 by Lee A. Hart. Created 3/15/2012. Last update 12/14/2013.
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