the cake is a lie, charge controller edition

Note:  we are discussing charrging 2x 125Ah AGM in parallel, so C==250Ah.   Dealing in general principles here – the battery mfg should be able to provide more precise info.  If they don’t provide it then they probably aren’t making real deep cycle batteries.

I've tried to get enough to ensure I end up floating charge

I come bearing unpleasant news.  Many/most controllers drop to Float too early, robbing the bank of desperately-needed Absorption duration. This means Floating doesn’t necessarily mean one’s batteries got fully charged. “The cake”, as they say, “is a lie”.

Lead batteries take around 5.5 hours to fully charge from 50% DoD. Depending on Bulk charge rate Absorption will be 4.5 - 5.25 hours. Unfortunately, most charge controllers drop to Float on a timer after 2-3 hoursNo bueno.

So how do we know when lead batts are actually fully charged? We watch the amount of current they accept at Vabs.   This is the acceptance rate.

Lead batts accept less and less current as they approach full charge. When AGM first start charging from 50% they might accept C/3 (250/3 == 83A). By the time they are fully charged they accept a relatively tiny amount.  This final rate is called endAmps, trailing amps, etc.  They push themselves away from the table because they are full.

Full charge is achieved when acceptance at Absorption voltage falls to between C/200 **(250/200 ==1.25A) and C/100 (250/100 == **2.5A) and stops falling. Lower current is generally “better” but each bank has its own personality.  Changes in endAmps over the bank’s lifetime tells you something about the health of the bank. Newer/healthier banks might get down to the C/200 rate and older/injured banks closer to the C/100 rate. As the bank dies it won’t be able to get down to even C/100 anymore and might stall at C/50 (5.0A) or something.  That power is  being converted into heat instead of increasing state of charge.  Time to start thinking about a new bank.


  • C/200 - my bank when new

  • C/175 - my bank after several consecutive days of excellent solar charging conditions (uncommon)

  • C/125 - my bank under normal use in normal solar charging conditions (common)

I will start thinking about new batteries when I can’t get down to C/100 anymore. The C/125 would worry me more if it didn’t rebound to C/175 when given the chance.