kshilanski avatar image
kshilanski asked

Questions about SmartSolar MPPT charging voltage and battery voltages


I would like to better understand more about how the SmartSolar MPPT 85/150 works. Last summer we had a SmartSolarMPPT 85/150 and a BMV-712 installed in our motorhome. We have 5 x 160w panels on our roof, and run 4 6v AGM batteries in a 12v system with a total of 440AH. The settings in the SmartSolar MPPT are custom set to Absorption 14.4v, Float 13.4, Equalization 15.5v.

I am still learning, so please forgive that some of these questions may be very basic. I really appreciate any help or ideas here.

In general, we feel like our batteries don't last as long as they should. So we're trying to test things and figure out if something is wrong with our batteries. We will also try and take better notes so we have something more helpful to share than just a vague feeling.

#1) I was looking at this document... And it says, "With regard to the Solar Panel voltage, note that the Solar charger will only operate once the Panel voltage has risen more than 5V above battery voltage." So do I understand correctly that if for example our battery voltage was 12.4v, and the voltage coming in from our solar panels was 17v, that it would not charge the batteries? In this example would the voltage from the solar panels need to be 17.4v or greater?

#2) I have a screenshot from a sunny day last summer. If I understand correctly, the voltage coming in from the solar array was 16.76v, and the battery voltage was 14.17v, and it says the battery was in Absorption stage. But since the voltage of the solar array was not 5v higher - would it actually be charging?

#3) We disconnected our batteries today. Right after we disconnected them, Battery 1 was at 6.29v, then after 5 hours of being totally disconnected it was at 6.23v. Battery 2 was at 6.30v, then 5 hours later at 6.23v. Battery 3 was at 6.31v, and 5 hours later at 6.24v. Battery 4 was at 6.31v, then 5 hours later at 6.24v. This will sound very silly - but I'm not quite sure what to make of these numbers! :) I am planning on calling the battery manufacturer tomorrow to ask. It seems like it's good that they were all pretty similar. We know the batteries weren't fully charged before we disconnected, and that we may have to do this again. If anyone has thoughts on whether these numbers sound good or bad that would be great. According to the battery manual, these 6v batteries would be considered full at 6.4v or more, and 6.25v is considered 25% depth of discharge.

#4) Is it possible to equalize our batteries too much? In reading the battery manual, it suggests equalizing them at 15.5v for 8 hours. And that you may have to do this 2 or 3 times. Can anyone tell me if it's dangerous to do that too much?

Thank you so much!!

smart solar set-up helpsmart solar charging behaviour
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kshilanski avatar image kshilanski commented ·

Sorry - the screenshot didn't seem to post correctly. Attaching it here. Thanks!

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adev avatar image
adev answered ·

Hi, I’m also not an expert so take my comments for what they’re worth, but I may be able to help bring a little more clarity on at least one of your points.

#1 - yes the victron MPPTs need to see +5v or more from your PV (solar) relative to the battery for a charge cycle to start. Once the cycle has started the threshold to maintain it is lower - I forget exactly but I think it’s 2.0v or or 2.5v. So long as it starts charging and the voltage then stays +2.5v it will keep going.

I would recommend re-configuring your PV array with groups of panelsin series/parallel rather than all in parallel. This will raise the system voltage so that you always have way more than the +5.0v threshold you need to start a charge (your PV voltage will be around ~35v or ~52v depending on how you configure the PV array, instead of around ~17v as now). The tricky bit here is you have 5 panels and you really need an even number to do that. So I would recommend either

a) buying another panel of the exact same spec as your other 5 and then wire them up in two groups of three. eg three panel in series - two strings like that and wire the two strings in parallel to the MPPT.


b) the same but using only 4 of your existing panels and either get rid of the 5th one or save it until you can add a 6th as per a) above.

#2 if it’s in absorption phase that means it has indeed been charging and is almost finished - see details above ;)

#3 Immediately after charging, your battery voltage will remain at the higher (than resting) charge voltages for a period of time - anywhere from a few minutes to a few hours depending on whether you have loads on. Normal resting voltage for a fully charged AGM battery is around 12.8v I believe (half these voltages for the ~6v figures), so if it’s any higher than that it’s either charging, or recently finished charging. It’s totally normal to see the voltage gradually drop after charging to around ~12.8 and this will happen quicker under discharge load.

4# yes it’s certainly possible to equalize too much, but I’m not sure about the ins and outs of that as I now use a lithium based system which is very different in that respect.

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kshilanski avatar image kshilanski commented ·

Hi Adev,

Thanks so much!

#1) Ok, so it sounds like the charge controller would need to see +5v to start a cycle, but once it starts, as long as the solar voltage is at least 2.5v more than the battery voltage it will continue charging.

You bring up an interesting idea about changing the solar panel set up. We will have to think about that. We originally did it all in parallel because our first charge controller was not MPPT, but now that we have the SmartSolar MPPT this might be the way to go. Our tricky bit may be that I don't think I can get that exact same panel anymore. I"ll take a look, and then run some numbers to see how things would go if we just did the 4 panels in series/parallel. Thanks for the idea!

#2) ok thanks!

#3) Good to know! Wanted to share that I just got off the phone with Lifeline batteries and chatted with a nice guy there. He thinks our batteries are about 30% discharged and sulfated from years of not fully charging the batteries back up. We've been learning a lot about solar. One of the things I've learned (through very expensive tuition!) is that just because someone says they understand how to install solar doesn't mean they do! Even if they have tons of great reviews :). We've had a series of installs, and then later that horrible realization that something wasn't actually set up right. So now finally, we're trying to take more ownership in understanding for ourselves how things work (which really we should have done from the beginning).

#4) Re: equalizing - in case this is helpful for someone later - In talking to Lifeline this morning, he thinks we'll need to get a whole new battery bank, but that we can try equalizing our current batteries a few times in hopes we can get through the season. I asked if it would be possible to equalize them too much - he said for our situation, 'no'.

Thanks again Adev for taking the time to answer my question! I'll look into the series-parallel idea with the panels. Thank you!

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adev avatar image adev kshilanski commented ·

you’re welcome!

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