I’m starting small, just learning how this all works. Here’s my plan.
A single 250W solar panel connected to a SmartSolar 100/30, connected to two Dumfume 12v 100ah LiFePo4 batteries.
Here’s my question. The batteries - series or parallel?
I asked Dumfume this question, here’s what they said, and I quote:
Hello, thank you for your inquiry.
The Victron Energy SmartSolar 100|30 controller is a 12V/24V automatic adapter model, and its maximum supported system voltage is 24V.
When two 12.8V batteries are connected in series, the system voltage reaches 25.6V, exceeding the controller’s rated voltage range.
This will trigger the protection mechanism, preventing charging from starting.
To ensure battery safety and charging efficiency, this combination cannot currently achieve normal charging.
Now, I’ll admit, I’m a newbie, but that doesn’t make sense. Everything I’ve read in the owner’s manual says the SmartSolar handles 24v batteries just fine. But… maybe VE is presuming a single 24v battery - not two 12V batteries in series.
It’s up to you as to whether you want your system to be a 24v system or a 12v system.
Dumfume (might be a clue in the name) can only advise you as to whether their batteries can be connected in series (not all can), they don’t know how other manufacturers’ products work - not sure why you asked them that? Certainly a random AI response from them though and, as is usually the case with AI responses, not grounded in reality. The AI saw “24v” in the name of the Victron component and then said “well 25.6v is more than 24v, so this won’t work” which while technically logical without any context, is wrong.
You should certainly ask them (or read their manual to make sure) whether their batteries are capable of being connected in series, as some BMSs will not permit that.
The controller can charge either voltage range, and it has no way to know whether a 24v bank is a single 24v battery or two 12v batteries in series or four 6v batteries in series for that matter - nor does it make any difference as long as everything is connected correctly. For more information about the controller and its operation, please see the controller’s user and installation manual: MPPT solar charger manual
Please also be aware that the controller requires vBat +5v from the PV side to start charging, so if charging a 12vNom battery bank, your solar array will only need to be putting out at least ~18v for the controller to start charging the battery, but if charging a 24vNom battery bank then your solar array will need to be putting out ~32v or better for the controller to start charging.
Thanks for your reply. Their manual says, “Series & Parallel Connections - 4 Parallel(Max) 4Series(Max).” So apparently, the batteries CAN be connected in either series or parallel. They also have pictures showing their batteries connected in both series and parallel. So, I guess their batteries are capable.
As I said, I’m a beginner. If I connect them in series, if the 1st battery doesn’t “need” the power, it’ll auto-magically go to the 2nd battery? Yes? I guess what I’m trying to ask is: The SmartSolar 100/30 manual says it’s works with 24v batteries. Does it care whether it’s one 24v or 2 12v batteries?
If they’re in series, there is no first and second battery, it’s a single 24v battery bank that you’ll be charging at 24v.
The SmartSolar has no idea how the battery bank is composed, it only sees a 24v battery bank.
Have a read through Victron’s free book Wiring Unlimited to start getting a handle on the physics, basics, and best practices of electrical systems: Wiring Unlimited
If you connect 2 batteries in series you MUST charge them both completely to 100% SOC individually before connecting them in series. Once in series they will charge and discharge pretty much identically as they will have the same current flowing through them. This is pretty much basic electrical knowledge.
In series all charging and loads are taken from the 24V connections, do not take a 12V load from the middle of the batteries as this will cause the 2 batteries to go out of balance. If you need 12V from a 24V system you need to install a 24V to 12V DC to DC converter such as a Victron Orion.
Remember that prior to the MPPT starting, the panel is open circuit so may be producing the slightly higher Voc to get 5V above the battery voltage. Once started you need to be 1V above the battery to continue charging. However, you may be very very close, a nominal 24V lithium battery could well be at 26.5V before charging starts so you need 31.5V to start. This is what Justin said, the 28V figure quoted by RL is too low for a lithium battery system.
The second panel MUST have the same Imp as the first panel or very close to it to be connected in series. If the two panels have different Imp then the panel with the higher Imp will only be able to deliver the lower Imp of the other panel, degrading its output.,