Why do battery needs a float current?
I mean, when it's full, floating and there are no loads? To fight the self-discharge, alright. But it draws a bit more than self-discharge would require. I have 300 AH AGM bank, self-discharge ...
Forum Discussion
Almot wrote:
My only guess is that battery becomes a space heater while on the float…
Correct! Your batteries, as well as the cabling & each of the interconnections, are resistors in the charge circuit. So, while a portion of the current is used to replenish losses due to self-discharge, the balance of the energy is being dissipated, in the form of heat... from every component in the circuit.
Using your example of 0.2A of current that you have not yet been able to account for, and assuming there are no other parasitic loads; It appears that the 2.7W (13.6V * 0.2A) is being "used" for heating (and electrolysis). That would indicate a total circuit resistance of ~68 Ohms (R=V/I, or 68=13.6/0.2) . Although this resistance value is much higher than I'd expect to see from healthy AGM batteries (I'd guess that'd be just a couple of milli-ohms each), it's within the realm believability for the entire circuit, especially when considering your use of booster-cables/clamps.
A closer review of your wiring (especially the crimped/soldered/bolted connections) may reveal some opportunities to reduce resistance and improve overall efficiency. But at the end of the day, since you are charging with solar, I wouldn't worry too much about the losses you've described.
FWIW, with my system (also 300A/h AGM's), with batteries at 100% SOC I typically see a float current of ~0.1A @ 13.6V (charger display indicates "0A"). Of course the resolution of my TriMetric is limited to +/- 0.1A, which hinders my ability (and interest) to monitor with greater accuracy.
Cheers,
-Mark
