Jan Steinman via EV wrote:
At that level, the battery has to dissipate only 400 milliwatts, which it should be able to do without damage under almost any circumstance.
You'd be surprised. A good new AGM or gel battery can easily go over 13.8v even with just 5-10ma. It takes a long time to get there (days), but it certainly will!
The amount of power the battery has to dissipate is the key here; not any absolute level of current or voltage.
Heating is only an issue when the current is relatively high. The problem with low currents is that it holds the battery above its gassing threshold (about 13.8 ). At this voltage, it is gassing, however slowly. The gas either goes out the vent (flooded battery), or slowly pressurizes the inside of the case (AGM or gel) until the pressure relief vents release it. Either way, you're losing water.
This is why a UPS or alarm battery that is continuously float-charged at just 13.8v will go bad in a few years. That's how long it takes to vent away its internal supply of water.
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