Hi, I’ve got a question about a smart battery charger for my ESS setup.
In short: I’m running an ESS based on a MultiPlus II 48/5000 and overall it works fine, no major complaints. However, on the DC side I also have some constant loads connected directly to the battery, which slowly but continuously discharge it. It’s not a huge load, at the moment the peak is around 600 W.
The problem shows up when there’s not enough sun to recharge the battery for normal operation. Right now the system is set to discharge the battery down to 75% SOC, with BatteryLife enabled, which effectively raises that to about 80%. The ESS behaves correctly and stops discharging at the set SOC, and all AC loads are then switched over to grid power.
The issue is that the DC loads keep discharging the battery outside of ESS control. Once the SOC drops about 5% below the set minimum, the MultiPlus starts the “recharge” procedure and charges the battery back up to the minimum SOC again. If there’s no sun at all, the battery just keeps cycling between roughly 80% and 75%, doing this maybe twice a day. I’d like to avoid that.
The system also has a SmartShunt configured as “DC system”, so the Cerbo GX knows exactly what the DC-side loads are.
What I’m looking for now is a charger that can be integrated into the system and controlled by the Cerbo, so it can continuously supply my DC circuits without having to discharge the battery and trigger the recharge cycle all the time.
So the question is: what kind of device would be best suited for this?
Why not simply let the mp2, which in itself is a ac to dc charger.
Any settings to control a separate charger can be set to control mp2 charger?
On another note why only soc to 75%?
I wish it were that simple. As far as I know, the MultiPlus II won’t work as a power supply for DC loads. Personally, I think there’s nothing missing hardware-wise — it’s more a software thing, either nobody thought of it or I just don’t know how to enable it.
The system behaves exactly as I described above. The DC loads discharge the battery down to the –5% offset from the configured SOC limit, then the Multi kicks in with the recharge mode and charges the battery at full power (70 A) back up to the minimum SOC. After that, the cycle repeats.
And why 75% in the first place? At this time of year we get so little sun that I can’t realistically recharge the battery by more than about 25% per day — and that’s already the absolute best case. That’s why I keep it at 75%, so that in case of a grid outage I still have some buffer available.
I guess what @1234enough is saying is, why not keep the batteries charged with your existing multi from the grid, so that the batteries ably power your DC loads?
The problem is that there’s no simple way to do this. I could switch the ESS to “keep batteries charged” mode, but then I lose the ability to store whatever solar energy I manage to produce during the day. It’s not a huge amount — in the current weather conditions it’s up to around 10 kWh — but it’s still something.
Secondly, I don’t want to waste battery cycles just to buffer power for loads that I know will always be there. If I know that at night I need around 150 W for DC loads, it makes much more sense for that power to be supplied by an independent charger / power supply rather than by the battery itself.
Thirdly, if I leave it as it is, I have no control over when the charging happens or how fast it happens. Sometimes the battery SOC drops by those ~5% in the middle of the night, and then the Multi kicks in and starts charging — noisy enough to actually wake me up.
In my opinion, this is quite a big oversight on Victron’s side: the Multi doesn’t switch to supplying the DC loads once the minimum SOC is reached. I would expect it to maintain that SOC using grid power. Especially since, in this state, the Multi isn’t really doing anything else anyway — it’s just passing grid power through to the AC loads.
This way I would lose backup power for those loads. They are connected directly to the battery primarily so they can run all the time, with absolutely no interruption — even in the case of a planned service or a failure of the MultiPlus II itself.
Don’t want to waste battery cycling, i find this augment slightly pointless.
Most lithium batteries have a cycle life for a full 0% to 100% every day for 10yrs or more.
I’m fairly sure there ain’t many who actually achieve this or even have thier systems set up to allow this.
Example: Eve specifications are 6000 - 8000 cycle life with charge to 3.65v / discharge to 2.5v, which most would agree is using cells hard and wouldn’t dream of such harsh parameters.
Personally i have discharge set to 2.8v / charge to 3.48v, as any more is well into the curve of the discharge/charge knee curve and wouldn’t give much more stored capacity.
Chances are most batteries will eventually fail due to age related degradation, not cycle life.
But hey this is what makes DIY great, it allows individuals to set up thier own systems how they want to and gives others ideas that may or may not suit thier own systems.
Maybe have a look at node red, this allows for personalised system settings for virtually any situation.
OK, I understand that this may not be important to you. For me, however, if something can be avoided, it should be avoided. Keep in mind that not every system runs on lithium batteries. There are plenty of systems still using lead-acid or AGM/gel batteries.
Another argument for looking for a different solution is efficiency. With a simple PV → battery → DC loads path, you lose at least about 20% in the best case. In my opinion, that’s worth fighting for.
The real problem, though, is that with the “DC System” option enabled, Victron does not maintain the configured minimum battery SOC. If the AC loads are already being supplied directly from the grid, then the battery should also be kept at its minimum SOC — but it isn’t, and that’s exactly why I’m looking for a solution to this issue.
So I’ll ask again: is there any Victron product that would meet my requirements? From what I can see, the Phoenix Smart IP43 Charger would solve my problem — but unfortunately it’s only available for 12 V and 24 V systems. There is no 48 V version.
To properly set up charging and discharging, you need to configure the shunt so that all loads from the battery are accurately measured. Once this is done, you can use Node-RED and DVCC to turn on the internal charger based on the SOC. It’s a straightforward process, toggle keep my battery charged.
The issue you’re describing is that the DC loads are depleting the battery below the minimum SOC, causing the internal charger not to activate in order to maintain the SOC, correct?
Another charger for 48V will be very expensive, and you will be better off just buying another multiplus if you just want to set one up for charging only.