Backchannel: AGM + bulk hole

from this thread:

I know it's overcast, so I'm not expecting much but I'm having a heck of time keeping my batts charged. I've never had this problem before. My PWM controller is only showing that I'm harvesting 1.2 amps from 400watts of solar. I've called around, and even on a day like this, I'm told I should be pulling closer to 10amps.

Before folks start jumping in with “OMG what you need is hundreds of dollars of my favorite brand’s gear” I’ll bring up a couple general thoughts.  They are not specific because I haven’t talked to OP yet;  walked the area for about an hour and didn’t see the van he described.

charging high end AGM

OP is running a Lifeline AGM bank, certainly one of the top tier manufacturers.  They should provide excellent value and service, assuming they are being charged Hard And Fast the way they like it.  Not my batteries so I’m not doing the legwork, but I remember a C/5 (!) minimum and C/3 (!!) recommended.  That would be 84A minimum to keep the batteries healthy.

These are not being charged Hard and Fast.  In this case there is 400w solar to 420Ah of capacity, a little less than the absolute minimum 1:1 rule of thumb for normal FLA batteries.  High end AGM would likely need 3:1 to get the charging hard/fast enough to keep them healthy.  I’ve collected some of Sternwake’s thoughts on this in the wiki.

PWM getting caught in a bulk “hole”

I’m not an MPPT evangelist, and I try really hard to help people make use of what they have rather than buying new stuff.   With that in mind let’s look at what is going on here.

The 30A rating of the controller is ok for the 400w array (assuming 25% derating), even though the array is  way undersized for the 84A requirement.

But the PWM isn’t going to stuff even 30A into the oversized bank because the bank is already undercharged.  PWM runs the panels at Vbattery at best, which is almost certainly a good distance from Vmp.  This means **once a PWM-charged gets into a hole like 50% DoD it is hamstrung and will struggle to complete Bulk charging. **

MPPT, which decouples Vpanel from Vbattery, would ram that 30A into the starving bank when it needs it most: during low states of charge.


In the immediate term the batteries need saving.   Irreversible hard sulfation is a real concern in this situation.  Hit the bank with a generator, shore, or alternator power to 80% state of charge and let the solar finish them off.

In the longer term the bank is oversized and doomed in this arrangement;  it cannot be charged to the manufacturer’s specification by solar alone.  A win-win would be to trade one of the Lifelines for a 30-40A MPPT controller.  That would put down the panels’ full potential (pun not intended) while giving the remaining battery more of a chance to live.  C/7 is not C/5 but is better than C/14 or worse.

IMO, of course.