Four types of Lead-Acid batteries concern us for vehicular purposes, automotive starting, low antimony deep-cycle, high antimony deep-cycle, and gel-cells. I'll go over characteristics of each. Generally, the storage capacity (ampere-hour rating) of a battery is a function of the surface area of the plates exposed to the chemicals.


Automotive Starting Batteries

It has one job only: to start your car. An average car uses more than 300 amps for a few seconds in order to start. The batteries are constructed with a large number of thin plates of lead sponge. This provides maximum surface area. The batteries handle only very shallow cycling, on the order of 1% in normal use. The starting battery will fail after approx. 100 cycles of 50%. Complete failure at 200 cycles. The sponge disintegrates with the repeated full charge and discharge chemical reactions. Lead particles separate from the plates and form micro-short circuits inside the battery. This highly increases the self-discharge rate. Maintenence-free batteries have added calcium to the lead sponges to harden them and reduce water loss. The calcium also increases the internal resistance, hence slowing self-discharge. The expected lifetime of a starting battery in true starting use is 3-5 years. In deep-cycle service, expect less than 2 years.


Low Antimony Deep Cycle Batteries

These are run-of-the-mill "marine/RV deep-cycle" batteries. It's a compromise between a starting battery and a true deep-cycle battery. They are much closer to starting batteries, however. The plates are somewhat thicker than starting batteries and have some added antimony. It is not designed for powering large loads for a long time. Deep-cycling will damage it, over time. In RV use, with usually no more than 20% discharge, the battery should last 200-400 cycles. If cycled 80%, expect a lifetime of less than 200 cycles, or about the same as the starting battery. The thicker plates and antimony add a bit of mechanical strength over the standard starting battery. In float service, the battery will last 5-10 years, much greater than starting batteries used in float service.


High Antimony Deep Cycle Batteries

This type is designed to be 80% cycled repeatedly for 5-15 years. There is almost no mechanical similarity between this battery and a starting battery. They are massive and huge. There are very few true deep cycle batteries with greater than 6 volts, as they would be too heavy to move by hand. The grids are over 4 times thicker than a starting battery's grids. And there is several times the amount of antimony in the grids. The plates are thick to add lifetime, not capacity. The plates are not constructed of sponge, but of scored sheets of lead with up to 16% antimony. The thickness of the plates combined with the high antimony content lowers the energy density, so this battery is heavier, larger and much more costly per kilowatt-hour. The case is also much thicker, and the plates usually leave a 1-3 inch space at the bottom to allow for accumulation of lead particles, so that they don't cause micro-shorts. The top of the case also has more space to allow for expansion of the electrolyte. Plates can be removed and serviced. As the cell interconnect straps are exposed, each cell's voltage can be measured individually. This allows the user to determine when an equalizing charge is necessary. Some batteries have "wrapped" plates, where perforated plastic is wrapped around the plates to keep the lead on them longer. Such a configuration add 25-35% to the lifetime of the battery. Such batteries are mostly used for electric vehicles, which force a fast 80% or more discharge. They are then recharge much more slowly. A 350 amp-hr 12 volt battery weighs 250 pounds and contains 4.5 gallons of sulphuric acid. They can be cycled 80% between 1000 and 2000 times. Lifetime should be 5-15 years.


Gel Cells

This type is designed for portability. They are small and have a jellied electrolyte. The case is sealed. The jellied electrolyte allows the use of this battery in any orientation. They are used often in aircraft and electronics. They are supposed to be clean and usuable where acid vapors and spills are unacceptable. They can be deep-cycled over long periods. They must not be charged or discharged too rapidly, otherwise it can gas, possibly blowing the sealed case. They are prone to sulfation if left discharged for a long time. With proper care, a gel-cell will deliver 1000 cycles over a period of 5 years or more.


Batteries and Temperature

As temperatures are lowered in a battery, ion mobility (the carriers of charge) and electron reaction rates are reduced. Thus most liquid batteries cannot produce the same amount of energy at lower temperatures. If the electrolyte freezes then ionic mobility will be lowered to the point that the battery is basically useless. This will probably not happen too often because batteries have awfull thermal conductivity so they never reach the ambient temperature. The other interesting thing about batteries is that at lower temperatures there is increased internal resistance. If you leave something small plugged in (like a radar detector or car alarm) it will draw current. This current will flow through this internal resistor creating heat. This in turn will keep the battery warm. But it will drain some juice out of your battery. Heating a battery produces the opposite effect. The battery will yield more. The only problem is the electrons tend to go crazy and cause the battery to self destruct. A boiling battery can produce a substantial amount of Hydrogen gas. Thus creating a small non-nuclear Hydrogen-bomb capable of burning you, your vehicle, and anything in its path. Avoid a boiling battery at all costs'



All of us are concerned about wiring our car properly. So much so that I'll bet most of you engineer to overkill. What are the issues you need to be concerned about? 1. Use the smallest reasonable wire size for the required current.


Wire is expensive and the larger you go, the more expensive it is. Wire is heavy and the larger you go, the heavier it gets.

  • Mechanically, smaller wire is easier to route, easier to protect, easier to fit connectors on and therefore, more reliable mechanically - up to a pratical limit - see below.

2. Use a large enough wire so there is no voltage drop. We want whatever it is we are wiring to operate at top efficiency.


3. Maintain an adequate safety margin. We don't want to melt any wires. The first thing you have to do is determine the current you have to carry. For DC circuits, that's relatively easy. Some equipment on a car is rated directly in current draw. Auxiliary fans, fuel pumps and things like that are rated in current draw - Amps. Some equipment is rated in Watts - mostly the lighting equipment. The power requirement in Watts will be printed right on the bulb or stamped in the base. To come up with amps use one of the formulas shown. Let's calculate for a typical 100 Watt Driving Light - the power required is 100 Watts and the voltage is 12 Volts - so the current requirement is 100 Watts/12 Volts = 8.33 Amps. Let's assume you have to run a wire 6 feet from a relay to the lamp and look at the chart on the next page. Using the 10 Amp column you'll find that you can run 10 Amps on 15 feet of 18 AWG with only ½ Volt drop. Go to the next size larger for safety margin and you're at 16 AWG. Now in reality, you have to balance the mathematical results with mechanical reliability and efficiency. For lighting, the rated output is figured at 13.5 volts, not 12 volts. With the 0.5 volt drop shown in the chart, you have 13.0 volts available at the lamp - and at that 95% rated voltage, you are only going to get 80% of the rated output - or the equivalent of 80 watts from a 100 watt lamp. In our example, I'd go to 14 AWG as the wire and connectors are physically stronger, easier to work with, and there's no voltage drop - plus I only buy three sizes - 14, 12 and 10 AWG. Those three and crimp-on connectors are readily available just about anywhere. And except for primary circuits, those three sizes will cover just about anything you want to wire in a car with an adequate safety margin. Is your Alternator big enough for all your electrical equipment? Each 100 watt lamp is going to draw about 9 amps so six of them is going to suck up about 55 Amps. The other accessories on your car - cooling fan, heater fan, ignition, fuel pump, running lights, etc. - are going to draw roughly another 30-40 Amps - your total power requirement will reach about 90-100 Amps. It's impossible to compensate for a small alternator by throwing in a bigger battery as the battery will just be drained and the voltage will suffer, affecting your light output and overall performance. Your best solution is to go to a modern, high output alternator of at least 100 Amps or more. If you are really worried about weight, you're better off with a smaller battery. All it really has to do is start the engine if the alternator is large enough to carry the rest of the load after the car is running.


Maximum Current load in AMPS @ 12 Volts DC
Wire Length in Feet
20 106532617 13
18 15075372518 15
16 224112563728 22
14 362181906045 36
12 5722861439571 57
10 908454227151113 90
8 1452726363241181 145
6 23421171585390292 234
4 37021851925616462 370
2 6060303015151009757 606
1 7692384619231280961 769
0 97084854242716161213 970



Maximum Current load in AMPS @ 12 Volts DC
Wire Length in Feet
12 15 20 50
20 ----- -
18 12---- -
16 1814--- -
14 302418-- -
12 473828-- -
10 756045-- -
8 120967229- -
6 1941551174623 -
4 3072461857437 -
2 50340330312160 30
1 63851138415376 38
0 80564548519497 48


Calculate the current load and find the next highest on the top row. Go down that column until you find the length you need to run. The wire gauge required is shown in the far left column. The maximum lengths are based on a ½ volt drop over the indicated length.


To be safe, always choose one or two wire sizes larger than you need for the indicated current carrying capacity and length. For example: You've calculated a 10 amp load over a length of 15 feet. The chart shows that 16 AWG is suitable (12A column). Choose 14 AWG to allow an adequate margin for safety.


Current-Carrying Capability of Some Common Wire Sizes
Wire Size (AWG) Continous-Duty Current *
8 46 A
10 33 A
12 23 A
14 17 A
16 13 A
18 10 A
20 7.5 A
22 5 A
* wires or cables in conduits or bundles


Resistance of copper wire per 1000 Feet at 25C
Gauge Diameter Ohms
20 0.032 10.35
22 0.025 16.46
24 0.020 26.17
26 0.016 41.62
28 0.013 66.17
30 0.010 105.2