spirou avatar image

should I extend absorption?

So I've been noticing this effect with the previous van and now again with the new one and I'm just wondering what should be improved. If anything.

The setup is this:

2x160W panels in series

100/30 smartsolar, abs. 14,4V, float 13,4V (+ temp.compensation)

2x 90Ah Varta LFD wet acid

(previous van had 1x120W, 75/15 and 125Ah gel)

The "issue" is when I leave the van parked at home and it sits unused for a while, I've been noticing the mppt isn't charging quite as much as it possibly should. When I'm looking at the history data I see a steady dropping min battery voltage numbers. Day by day it goes down by 0,01-0,03V. In itself not much but over the past 16 days the min voltage came down from 12,95V (normal resting voltage for Varta LFD) to 12,62V. Obviously the BMV shows 100% SOC but that's another matter altogether.

I should also mention there is a constant standby load of about 1W (mppt, bmv, gas alarm etc.) which cumulatively add up to about 1Ah per night.

Each day the mppt goes normally through all phases, reaching anywhere from 10 to 40W with 30Wh daily yield over these past 16 days.

I've turned off adaptive absorption duration as it sometimes switched directly to float. I've also increased from 1h to 2h fixed duration but there is no change to this downward trend.

The thing is, if I use shore power/EHU/whatever you want to call it and use the CBE mains charger, it will charge at 14,1V (abs. duration 1h30), float 13,8V, then unplug in the evening, the min voltage next morning will still be over 12,9V as I'd expect.

Like I said, I've noticed the same behaviour on the previous van which never registered any (minimal) standby drain on the BMV. That gel battery also had a high resting voltage (13,2V).

I know I'm just nitpicking but it's something I'd like to get to the bottom off at some point. I'd like to figure out why the mppt doesn't charge "enough"? Why isn't the min battery voltage in the same 12,9ish range every morning if all the charge stages are completed normally? I'm hesitant to just keep increasing the absorption duration, or voltage.

MPPT - Solar Charge ControllerVRM
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1 Answer
JohnC avatar image
JohnC answered ·

Hi @spirou

What you may be experiencing here is batts in storage rather than cycling. Like the 12.95 standing V you may be familiar with is just indicative of the time they may need to complete the chemical reaction. Your minor load/charge won't interfere much with that, and I'd be happy to see 12.6V in the morning after weeks in storage.

Forcing more charge either way is likely in 'overcharge' territory. The batts will react to that, but may too be causing them harm.

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OK, but assuming I use the van at this point, without plugging in before leaving, I'll end up somewhere around 12,2V after a typical winter night out with heating, fridge and other miscellaneous use. Unloaded that's probably 0,15V higher but still what I consider quite low after a single night. It would be acceptable after 2 or 3 days normal use and little or no solar/alternator charge in between.

I understand battery chemistry is complex but if I'd take full batteries out of the van entirely for the same number of days they'd probably still be well above 12,8V.

Today I added another 30min for a total of 2h40min absorption duration (~3h bulk, ~3h float) just to see what happens. Yield went from usual 30 to 40Wh.

Yeh I hear ya. But really, 30-40 Wh/day is tiny, and indicates your batts are fully charged. They'll self-discharge to a degree, and to top up a small load won't need much. To try to force more into them regularly can likely only lead to reduced life.

I appreciate you have your mind set on a high standing V in the morning. But your wets aren't gels, and you'll never see that unless you're cycling them and charging heavily while the sun is up. And it's only because of incomplete chemical reaction, perhaps 'dissolution of surface charge' might be an appropriate term.

What's best for you might actually be in the opposite direction, a Victron charge algorithm that spends most of it's time in a 13.2V Storage mode while you're not daily-cycling.

I would agree completely if talking about regular wet cells like my starter battery with their 12,6V when full. Varta LFD really does rest at 12,9-13.

Anyway, this morning I found it at exact same minimum as the day before. First time in 17 days when there was no decrease. But, for experimentation sake I changed settings to 14,2V @ 1h30 and 13,7V float (it was 13,3, not 13,5 as reported before). Maybe I get to the same place with less agressive high.

Went for a look to see why your batts might be so 'different' from other fla's. Nothing in the sales blurb suggests anything special.

Note though that there's the odd maker I've seen who've stated their rated Ah won't be reached until they've been in use for awhile (ie broken in). Possibly a reactivity thing related to your experiences.

You could even play with a hydrometer if you doubt they're fully charged, but that comes with issues too.

If you can reach a stable tail current of (say) 1% A vs Ah at Vfloat each day, then they're charged. (ymmv). And the standing V they settle to you'll just need to learn to accept.

Also commonly, a Victron charge algorithm in Storage mode might give them a stir-up every so often. Or even an Equalize. But you don't need to do this every day by overcharging them..

Varta are notoriously scarce on technical details but this battery is quite popular with UK motorhomers and it is universally accepted as having a high resting voltage. Mine came at 12,75 out of the shop and they were at least 2 months old at the time.

Also, it's sealed so no access for experimenting with hygrometers.

Anyway, this guy had much more experience with them than me and mentions 12,97V

The secret sauce seems to include higher float voltage. This morning was the first time ever the min V was higher than the day before. Onwards with experimentation...

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