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Battery Temperature Compensation

Introduction

Performance is not usually economically viable. As many engineers can attest, `it's not performance, stupid, it's cost'. That is, after all the discussions of why some performance issue is important, the final decision is to go with the least-cost design. Often the least-cost design doesn't really work too well, but as long as cost is the motivating factor behind a product, performance isn't offered by anyone who expects to be competitive in the marketplace.

Be that as it may, Ample Power introduced the first alternator regulator that was temperature compensating in early 1987. The sensors were not very installation friendly, but, once installed, performance was superior to any other regulator.

If temperature compensation is so important, why is it not universally offered? If this were a computer program, we'd tell you to go to the top and start reading again. Temperature compensation is important for battery longevity, particularly for sealed batteries. It isn't universally available, because Ample Power is ahead of the competition, and isn't afraid to produce products that perform, even if they do cost slightly more.

Universal Physics

An interesting phenomenon happens in the physical world. Many `things double every $10^\circ $C.' Before you conclude that this is a hen without lips talking, let's elaborate. If a particular mechanism has a determined failure rate at $25^\circ $C, then the failure rate will more than likely double at $35^\circ $C. A transistor that has a specific leakage current at $50^\circ $C will exhibit twice the leakage at $60^\circ $C. A battery that forever accepts 5 Amps of charge current at 14.4 Volts and $77^\circ $F, will maintain 10 Amps at the same voltage if the temperature is raised to $95^\circ $F.

Now consider this. Power is equal to Volts times Amps. For 14.4 Volts and 5 Amps, power is equal to 72 Watts. If we double the current to 10 Amps, power is equal to 144 Watts. Have you ever noticed the difference in the heat generated by a 75 Watt light bulb versus one of 150 Watts?

Thermal Run Away

In a typical situation, a vessel or vehicle will have a high capacity alternator and a limited capacity battery bank. During the bulk charge step, the battery can accept most of the alternator current and convert it back to available capacity. Once the battery nears a full charge, excess charge current becomes heat. Small at first, the heat begins to accumulate in the mass of the lead plates. As the heat accumulates, temperature of the battery begins to rise. That means ...yup!, current through the battery begins to double for every $18^\circ $F! But wait ...that means more power is dissipated in the battery which means more heat is generated, which means more current flows, which means more heat, which means ...double trouble! If you're lucky you won't be looking at the battery when the caps lift off into outer space with acid following in close formation!

How Common Is It?

When alternators are small compared to the battery banks being charged, and regulator voltages are typically low because they are designed for starting batteries, thermal run away is not much of a threat. It only becomes a threat when the size of the alternator is larger, and regulator setpoints are higher to achieve a full charge. We know that thermal run away happens from conversations with cruisers who report batteries too hot to touch. We expect more in the future as more high voltage regulators are sold without temperature compensation. Thermal run away may be the reason that those four month old batteries are no longer any good. It may explain why you only get 3-4 years from a set of batteries; why you add water on a regular basis; why gel batteries don't give you superior service.

The Thermal Run Away Solution

Prevention of thermal run away is easy. As the battery begins to heat, reduce its terminal voltage. This defeats the doubling effect of charge current. As the voltage goes down, the battery will accept less current. That means less heat buildup. It also means longer battery life and less electrolyte loss.

The Ample Power Solution

While the Ample Power solution is more costly due to its battery temperature sensor, it works `like a Swiss watch.' The temperature sensor attaches to the positive post of the battery, typically the hottest point in a battery under charge. As the battery temperature rises, the Ample Power regulator decreases the charge setpoint. The temperature rise may be from daily or seasonal variations, or from the mere fact that the battery is being charged. The net result is a perfect charge.

The following table shows the voltage applied for any given temperature. For intermediate temperatures, extrapolate between the values in the table. (Gel batteries, as well as AGM batteries are slightly different.)

Temperature F/C Absorption Voltage
122/50 13.80
104/40 13.98
86/30 14.19
77/25 14.34
68/20 14.49
50/10 14.82
32/0 15.24
14/(-10) 15.90
(-4)/(-20) 17.82

While any standard 2-step regulator can charge batteries, only an Ample Power temperature compensated multi-step regulator can provide a fast, full charge and also extract maximum battery life. For a nominal difference in price, you can save many times more in ultimate battery cost ...not to mention peace of mind.