I want to add more battery capacity to a system. The goal is to have it behave as if there is only one battery, to the user. So, no manual handling.
The system consists of
3 Multiplus II 8kVA in 3-phase,
two MPPT chargers with about 9kW peak of PV panels,
DC-loads,
a DC source of maximum 5kW
a 48V/668Ah (20h) traction battery set with a Smart Shunt 1000A
a Cerbo
The battery is too small to store all the energy that is generated in the summer during the day. (because last year’s upgrade of adding 7kW peak PV panels). So, I want to add another traction set of 48V/922Ah, a second Smart Shunt 1000A and a I/O extender.
I had the idea of switching the batteries with two high current switches (>700A). The conditions to switch will be decided by the Cerbo on SOC and current conditions.
For discharging, the idea is to switch to the other bank when a SOC difference is measured between banks of more than 5%. I made a test program in Node-Red and that seems to work fine. (It also checks for maximum current). I also was able to switch between the active Smart Shunt with Node-Red depending on the active battery.
For charging, I thought that it might be more effective to first charge both batteries to 85% and then continue with the remaining charge (which I understood is less efficient).
I wonder how the Multi’s will react to the changing of the battery during charging:
· Will the Multi’s continue in bulk when during that phase the battery is changed, the voltage and the SOC value make a drop?
· When will the Multi’s go to absorption and for how long?
· Is it maybe better to first fully charge one battery and then the other?
· Is it than possible to force the Multi’s into bulk again or will it go into bulk due to the suddenly lower voltage after switching to the other bank?
Another issue is how to cope with the temperature, if I connect temperature sensors to both Smart Shunt, will the system use the temperature of the active one?
I’m no expert, but disconnecting / reconnecting 3x mp2 under load/charging isn’t something i’d recommend.
I feel the switching contacts may not last very long.
Why not simply parallel the 2 battery’s, system will simply see this as one.
Sometimes you can’t add newer batteries to an older bank. So i do see where this separate bank idea is coming from. But - no- switching live it not a good idea.
One safer solution could be to have a second smaller 3 phase inverter system and set that to feedback to the first during low battery soc events to offset load and possibly back to the battery bank over all phases.
We have something similar set up with node red. It also controls when the system recharges on excess pv and lower load conditions from the first system.
The old battery is 3 years older than the new one. In contrary to LPF batteries parallel switching of traction batteries is not recommended. It seems that the weakest will determine the performance.
I have found isolators that can switch 350A (maximum 1000A) and put two parallel for each battery (so 700A mac 2000A per battery), first close than open, so the load will always connected.
The program checks if the current (in or out) the battery is lower than 50A.
I did a test and measured the current from one battery to the other after connecting them together, these currents stay below 50A. (I think that is because of the nature of the lead-acid type of battery, that does not accept big charge currents?)
Indeed, because of the difference in age and capacity, it seems not recommended to switch the two banks in parallel.
What do you mean with live switching? The Node-Red program checks the current and only switches (disconnecting after connecting) if it is below a threshold of 50A (about 2500W). I think for charging it even can be lower, because with the old battery the charge current dropped to <30A within 2 hours of bulk. Do you think this current still might be too much? Maybe I better go for draining one battery completely before switching to the other and the same for charging? I guess than the Multi’s will see a low voltage as a trigger to start bulk again?
Nevertheless, where can I find more information about the system you mentioned?
It is a custom solution we used for a client with a similar issue. It was lithium based though. They weren’t able to add to the bank as intended because of a manufacturer supply issue. But didn’t want to loose their investment either.
It is basically one system attached to the ac2 out of the first system with only the ac input connected. And set up to feed back in at a certain time. when the main bank has reached a certain level. And then charge up after the main bank has reached 98%. With lead based you would probably have to change that a bit. It doesn’t even really need to be a three phase unless you have high loads.
SOC tracking will become interesting for sure switching between the two banks, you will also potentially get a high discharge from one bank into the other (they will be at different voltages and resistances.) that is what will make the switching potentially dangerous. Never mind switching under load which may make dc ripple issues (and then powr pack damage i imagine.) maybe somoen has done this but i see too much potential for disaster.
You will need pretty darn good beefy ignition rated DC contactors. Remember there are also gasses in the room and with discharge and a change under load could set off unintentional things.
I don’t understand why your proposed solution (which makes a lot of sense by the way) has to be 3 phase. Can’t just be single phase feeding the other battery?
It can be either single or three phase, it depends on how the system is loaded and how much energy need/storage there is. (It is manipulating the same mechanism a single phase PV in erter works on a three phase system.)
Power gets shared over the DC bus to the other phases of the charger is enough on the phase and th inverter feeding it can invert that backwards. (which is why one central bank is an advantage).
I do not know enough about the system use and needs to be specific and neither should I be more specific over the forum.
The set up allows a controlled charge and discharge from both systems. It can even be set up with DESS or scheduled charge in ESS or a combination of both. The options are almost endless.