A have two 18V solar panels and two batteries 12v and 24v to charge on my boat (no controllers yet).
Which option is the best:
To connect panels in serial to get 36v and connect two controllers on the same 36v array in parallel (one for 12v battery and another one for 24v)
Two panels in serial, one 24v controller and then in parallel 24v battarey and 24v to 12v DC-DC converter/charger for 12v battery
Two controllers, one for each panel, one for 12v and one for 24v with 12v to 24v DC-DC converter
1&2 are preferred for me, as 24v battery has higher power consumption.
You can’t share PV arrays between MPPT’s.
You need more than 5V above the required charge voltage to start the MPPT’s, with only 36V available from the panels in ideal conditions, performance may not be great.
So options 1 and 3 won’t work.
I have experimented with this but both batteries were the same voltage 12V. What I found was the larger 100/30 took more power than the 100/20 but both worked as expected. I wouldn’t listen to anyone try it. The 24v to 12v DC-DC converter will be very inefficient on a small system.
It is completely unsupported to do this and is also documented as such. It causes conflicts with the trackers, advising people who are not familiar with the environment to tackle completely unsupported configs without understanding the risks is ill advised.
As a Victron forum we cannot advocate tackling something that a member should not be doing.
Noted. None of the options listed are ideal. If it works, it works—I only did it because I had to send back a 100/50 and had a 100/30 and 100/20 lying around. You could make it work manually with some solar isolators if you want it documented, but the smallest DC-DC converter you can get is 12 amps. Ideally, you’d want an isolated one for stepping down from 24V to 12V. The panels are likely 100W to 200W max. Honestly, it would still end up being a bit of a mess.
Indeed. There are members doing some pretty creative things, some times out of necessity or because Mr Google said it was a brilliant idea
We always strive to educate, highlight the risks and nudge towards best practice where possible.
When it doesn’t meet expectations, they inevitably end up back here unamused.
In 1. two controllers are in parallel, so each gets 36v
In 3. I get 2 separate systems:
A) 1 solar panel - controller - 12v battery
B) second solar panel - controller - 12-24v converter - 24v battery
What could go wrong? At least in 3.?
guystewart
(Guy Stewart (Victron Community Manager))
8
Understanding why this will not work reliability needs a bit of technical understanding on how an MPPT works.
To put it simply there are two jobs for the MPPT, one is to get the maximum amount of power from the solar panels possible to charge the battery.
The other is to stop the PV panel from producing power when the battery is full.
The way the MPPT stops the flow of power from the panel is by adjusting the panel side voltage.
If you connect two MPPTs to the same panel (or string of panels), then the panel power output will not behave as they each expect.
In the best case you get a very inefficient system when only 1 battery is ever charged (because one MPPT will reduce panel power when its battery is full).
I won’t go into the worst cases, because that should be enough to determine it is not a good idea and there should be a different approach (such as a dedicated DC to DC charger like the Orions).
Without seeing the full system, or understanding the full requirements, both of which could change this advice, I would suggest running both panels in series (36V) into the 24V battery, as it’s the most efficient use of the MPPT current limit (20A or whatever it is is 240W at 12V, and 480W at 24V). And then get a 24V to 12V DC to DC charger.
Thanks! I got it.
but nevertheless one 18v panel, one controller, one 12v to 24v converter should work fir 24v battery I suppose.
guystewart
(Guy Stewart (Victron Community Manager))
10
The PV panel voltage must be a minimum 5V higher than the battery voltage. And the bigger the difference the more efficient it is, so even 36V isn’t perfect, but it will work.
An 18V panel will not charge a 24V battery with a Victron MPPT.
Why would chose this 12 → 24V arrangement over the 24V → 12V one?
That will work, but ensure you get the 12v to 24v DC-DC charger, not the converter. The converter is power supply only and you can get the non-isolated if the chemistry is the same. The dc-dc chargers are extremely inefficient and if you can tell us what size the panels are, it could help with the size of the DC-DC charger.
150W 18V each.
I will better connect 2 panels with 1 controller to 24V battery only.
12v one will be charged by boat engine.
In addition I would like to charge 12V battery from 24V one with 24v-12v DC-DC charger when engine is off - but I’m not sure if I can just connect that charger and engine charger to the same battery simultaneously…
