I recently upgraded to a Victron MPPT 75/15 solar charge controller. My solar panel is 20 watts, 12 volts. I have a 12 volt, 80ah AGM battery for my sailboat (to power accessories) and I’m just using the solar panel to keep the battery fully charged over the winter. The accessories are all turned off at the circuit breaker so there should be zero load on the battery. However, the controller can’t seem to keep the battery fully charged. I’ve posted screen shots of the Victron app showing the charging state over the past 13 days. Prior to hooking up the solar panel (13 days ago) I had fully charged the battery using an AC trickle charger up to about 13.8 volts. You can see from the screen shots that the battery voltage has been steadily dropping, as low as 11.93 volts today. Have I got a bad battery? Any ideas?? Thanks!!
My guess would be the charge controller is using more power than the panel can supply. Mppt 75/15 has a 25ma self comsumption, which works out to 0.3w / 7.2wh per day.
Some days the panel produced zero wh.
Bigger panel required.
Also pv voltage some days didn’t reach the required +5v above battery voltage to commence charging.
Bigger panel with higher output voltage required or another same as panel in series required.
Thanks! Before buying a bigger panel and after thinking more on the matter, can you tell from the app screens whether there’s a chance that a phantom load on the battery is causing the problem? I’m wondering if it’s worth crawling into the boat and disconnecting everything from the battery except the solar charger leads and see if it makes a difference.
Yes there could be a phantom load, always worth investigation.
But going by screenshots, the self consumption of the mppt is more than the panel can supply.
OK I verified that there was a phantom load, I measured .34 amps, so about 4 watts!! Turned out to be a marine radio that was turned off. Anyway, I now have everything disconnected except the Victron charger. Now, on a sunny day, the readout on the Victron app shows the battery voltage varying from 13 to 14.2 volts. On a dim day when the panel is basically putting out zero watts, the battery will sit at 12.52 to 12.58 volts. I’ve read that 12.5 volts is about 80% charge level. It seems like the charge level should be near 100% if it’s only been resting for a day or two. Does this mean the battery is starting to fail and needs replacement? The battery is a name brand and less than a year old.
I would also lift the panel up to get better sunlight so it is the highest thing.
Your picture shows snow, so low temperature, battery voltage is lower when colder. All the charts refer to normal temperature. Even named brand AGMs will fail early if you undercharge them or leave them.for periods with fully recharging. My guess is it is temperature related.
Thanks for the input. I just bought this battery last May. All summer, when I brought my sailboat back to a mooring after a day of sailing, I would hook the battery up to the solar panel charge controller. I’m sure the power output of the panel was sufficient when the sun was shining, but I’m thinking that when there were several cloudy days in a row and that phantom load was using 4 watts continuously, the battery probably got drained to 50% or less several times and sat there at the low charge level. I suppose this may have caused sulfation to occur? I have an automatic charger that has a “repair” mode. Do you think I should remove the battery from the boat and run this repair cycle on it to see if it helps? The charger is this one: Amazon.com: 1.75-Amp Car Battery Charger, 6V and 12V Smart Fully Automatic Battery Charger Maintainer, Trickle Charger, Battery Desulfator for Car, Lawn Mower, Motorcycle, Boat, Marine Lead Acid Batteries : Automotive
Wont hurt to take battery off and put it on another charger.
I’d take it off put it in house overnight to warm up, then put it on plug in charger.
As its sitting at 12.5v, i would say its still well usable.
As above if panel can be lifted and angled more vertical may help stopping snow sitting on it.
Good you found unknown load, every little helps.
You can give it a try but once sulphate, hard to recover AGM batteries as they do not like the high voltage recovery charge, if you boil off electrolyte and it vents it can not be replaced.
Thanks for all your help guys, I think I finally have an understanding of how to proceed.
OK, me again haha. Well a month ago I ran it through the “repair” cycle with my AC charger, then I hooked it back up to the Victron (all other loads disconnected) and forgot about it for a month. Today I looked at the “history” graph of it and I’m posting the picture below. Does this look healthy when it drops to 12.1 or less every day?? There’s no way the tiny load of the solar controller itself could be draining it that quickly.
