question

whiskeywilderness avatar image
whiskeywilderness asked

Just got our multiplus 24v 3000w 50amp inverter and are getting multiple issues

We live in a Skoolie… System is currently connected to shore power - our system has 110v fridge which pulls 400w per 24hours and 12v lights - couple plugs to charge our phones/iPads - we use way less currently than the system is set up for. But we plan to add a heat/ac in the future.

Inverter is in absorption mode - set to 8 hour time, is that appropriate?

We have 8 -270ah lifePo4 wired in series to create 24v battery

We got a dc ripple alert and the inverter shut off. We turned it back on then off and on again based on the basic troubleshooting.

Our whole system is the Bluetooth connection systems from victron -

It says charged to 100% - 26.6v -

We’ve only had the system up and running for 2 days.

Could really use some insight. Just really confused after two days of it working perfectly.

inverter current drawdc system
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3 Answers
Alexandra avatar image
Alexandra answered ·

@Whiskeywilderness

8hour absorption time is too long for lifepo4. Set it to one hour. The battery bms seems to be unhappy. What are the recommended charge voltages? And are the cells all balanced?

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whiskeywilderness avatar image whiskeywilderness commented ·
Can you elaborate on what you mean by charge voltages? For what part? Battery? Bms?
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snoobler avatar image
snoobler answered ·

Did you top balance your battery cells before deployment?


What BMS?


LFP requires very little absorption time. The only time you want a long absorption time is if you're trying to restore balance in an unbalanced battery. 1 hour should be sufficient.


If the BMS is triggering high voltage protection due to cell over-volt, I can see the MP reacting with excessive ripple.



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whiskeywilderness avatar image whiskeywilderness commented ·

The batteries are all balanced and now the inverter of turning off and on, every 3-4 minutes.

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snoobler avatar image snoobler whiskeywilderness commented ·
Did you purchase "matched" cells, or did you actually go through the process of top balancing the cells?


What BMS?


What absorption voltage?


What are the cell voltages doing when the MP is cycling on and off.


This sounds like classic cell imbalance and BMS cut-off.

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Trevor Bird avatar image
Trevor Bird answered ·

@Whiskeywilderness the following may be a plausible story. The internal BMS of the battery may be switching cells in and out creating instability of the terminal voltage. This is misinterpreted by the MP as DC Ripple and it shuts down. This can happen more so for series strings because a BMS does not exist across the entire battery bank, only the individual batteries. Batteries can charge at different rates so voltage imbalance can occur across the batteries and across the individual cells within the batteries. To stop this happening lower the absorption voltage the charger is set to. Lower it by 2 volts to start with and see if the instability improves. Run it like that for a while and if you want to raise the voltage a little but not up to where it was. Cell balancing refers to each cell in the battery having the same voltage. If the internal cell voltage reaches 3.75 volts the BMS disconnects it to stop it overcharging as a protection mechanism. Each 12 volt battery has 4 of those so the BMS tries to keep them all “balanced”

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whiskeywilderness avatar image whiskeywilderness commented ·
We have a 24v 200a Daly BMS on the batteries.
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snoobler avatar image snoobler whiskeywilderness commented ·
And did YOU personally (or pay someone to) charge all of your cells to 3.65V in parallel prior to deployment as a 25.8V battery?


If the answer is "no," you need to top balance your battery.


Alternatively, you can lower the absorption voltage until the problem goes away. I would start with 26.4V and increment up by 0.1V until the problem returns, then back off 0.1V. Leave it at that level for a week or so and then increment up by 0.1V until the issue returns, drop back 0.1V, run for a week, etc.


If it's a Smart DALY, configure balancing for 3.38V, 20mV and during charge only.


IMHO, your problem is because of an imbalanced DIY battery that wasn't top balanced is tripping the over voltage protection due to one or more cells exceeding the cell upper limit, which appears as DC ripple to the MP or results in charge cutting in and out cyclically.


I have personally seen batteries with as little as 0.1% cell SoC imbalance trigger BMS protection.


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whiskeywilderness avatar image whiskeywilderness snoobler commented ·

We top balanced them and checked them repeatedly before starting the system up. We checked them again and they are all the same. We also restarted our bms and did a charge volt reduction of 2 volts recommended by another here and it seems to have done the trick. Will update after 24 hours

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Trevor Bird avatar image Trevor Bird whiskeywilderness commented ·
@Whiskeywilderness I have had a similar problem with batteries with the Daly 100 amp BMS so I understand what you are experiencing.
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whiskeywilderness avatar image whiskeywilderness commented ·

So we did what you recommended and reduced it by two volts as well as resetting our bms and that seems to have done the trick. Time will tell.

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snoobler avatar image snoobler whiskeywilderness commented ·

You said you "top balanced them," but you didn't answer the question:


Did you charge all 8 cells in parallel to 3.65V?


Checking cells in the 3.2-3.4V range tell you nothing of their balance. Even cells with notable difference in SoC will read nearly identical voltages due to the flatness of the LFP curve.


If you did charge to 3.65V, and if there was ANY delay between top balance and deployment/operation, it's very common for cells to drift out of balance and still meet datasheet specifications. I witnessed this happen on a build where 7 days elapsed between top balance and use - they were out by 0.28Ah and couldn't be fully charged to 28.8V w/o tripping the BMS.


Lastly, with all builds, it's a good idea to triple check all connections for proper contact/torque - including cell interconnects and crimp quality of the BMS sensing ring terminals.

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