question

mattguy avatar image
mattguy asked

voltage spike during absorption on multiplus 3000 12v with LiFePo internal BMS

Hey folks, I have a similar set up to the post here.

The main difference: I have is a single large 360amp/h battery from expion also internal BMS.

Using the latest Multiplus firmware applied via the android app.

I'm seeing similar behavior where the multiplus will bulk (90amp limit) to 14.2 (my setting) and start absorption. tested with discharge to 95% so I'm not watching the output all day.


The voltage will then creep up to 14.3-14.4 (multiplus perspective) which is above my set desired set voltage (confirmed with multimeter). Near the highest sustained voltage I see a similar amp spike as the related post followed by a voltage spike (~16.1v) for less than a second. (See attached video of the system return to pre spike voltage). Note the V drop from the multi plus to the battery is 0.08-0.09v.

The manufacturer has confirmed the BMS will cell balance at top of the charge and has recommended 14.2 - 14.4 for 1 hour in absorption and doesn't believe I should be seeing this behavior.


  • They suggested removed all temperature cables. Temperature is provided by the lynx shunt, and lithium profile is selected for no temperature comp on the multiplus. Can this be the issue anyway? I would rather not remove the temperature sensors.
  • Would lowering the voltage further have a benefit e.g. 14.1 vs 14.2? as it seems the multiplus allows voltage to exceed the settings.
  • Out there idea: any issues fitting an Alternator protection device in series to prevent these voltage spikes if the BMS is exhibiting normal behavior?

Next steps I'm considering

  • Call the battery manufacter again and confirm cell balance behavior
  • Repeat testing without multiplus and let mppt do the charging to see if the issue is isolated to the multiplus. Note: during the above discription the solar panels were off.
  • Lower voltage to 14.1 if acceptable
  • Other...??? Any other ideas from the community to test?


battery chargingLithium Battery
signal-2023-03-18-16-01-08-417.mp4 (3.0 MiB)
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4 Answers
snoobler avatar image
snoobler answered ·

If your battery BMS engages over voltage protection and cuts off charging, the charger votlage will spike. This is extremely common.

Recommend you set absorption to 13.8V with a 4 hour duration. Float to 13.5. See if you stop getting the spike.

The creep up from the absorption voltage suggests the MP is not charging as programmed, temperature compensation is enabled, or there is another source programmed to a higher voltage.

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mattguy avatar image mattguy commented ·

Thanks for your reply. There are no other charge sources. Solar panels are off with dual pole disconnect. I used a large load to discharge the batteries without waiting longer than a handful of minutes before recharge. Could that have caused this?


I will also add these batteries recommended charge is 14.4-14.6 and BMS cutoff at 15.6 so even then creep to 14.4 should be okay.


I'd have to check with the manufacturer, but I don't believe that 13.8 is high enough for the cell balance.


Temperature compensation is off according the the below. Or there is a bug in this FW.


smartselect-20230318-222919-firefox.jpg


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thanar avatar image thanar mattguy commented ·
Balancing kicks-in depending on individual cell voltage, not pick voltage. Hence, if you set the whole battery voltage to 13.8V and a single cell is almost full, it will get balanced out. However, as the battery gets balanced over time, you’ll have to adjust battery charging voltage to a higher setting.
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mattguy avatar image mattguy thanar commented ·
Thanks for your reply. Can you elaborate. Why, over time, would I need to increase voltage to balance the cells when charging with a lower voltage (e.g.13.8 - 14.0)?
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Alexandra avatar image
Alexandra answered ·

@mattguy

the other thing that can cause the bms to switch off is charging too fast for the cells to balance. So a slower charge in amps may help. The mppt charge may be gentler.

They are new these kinds of issues are to be expected. A drop in charge in settings voltage of 0.2v should be ok as well. At least while you are 'breaking them in'.

As long as you have customised voltages and charge current using a lifepo4 charge profile, they will charge correctly. @snoobler brought it up because often people skip changing the charging battery type in the profile. So no firmware issue there.

Are you sure your 12v battery can take 90A continuous bulk charge? What is its C rating?

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mattguy avatar image mattguy commented ·

@Alexandra, thanks for your reply! I'll try even lower bulk voltage tomorrow and then solar charging to see if there's a difference. Bellow is the original battery spec sheet for the battery. I say original because the newer model has slightly different voltages. Namely, lower recommend float, and lower discharge currents. The charge current is the recommended current, but perhaps it was too much for a mostly full battery?

smartselect-20230318-234122-mj-pdf.jpg

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Kienan avatar image
Kienan answered ·

Lithium batteries (LiFePO4) have the longest life if they aren't charged above 13.5V too much, but cell balancing is important, so I'd check with the manufacturer to see what is the minimum voltage where balancing can occur. With most brands, you never need to charge it above 13.8V. Also, if the BMS tracks State Of Charge (SOC) then you need to charge it high enough to reset the SOC to 100% consistently. Then I'd set the charging voltage, then watch it. If the voltage creeps up above that voltage, then I'd adjust the voltage lower until it consistently charges to the desired voltage and no higher.


@thanar is correct too. If the pack is really unbalanced, then the fullest cell will reach it's critically high voltage and the whole pack will shut off (block charge current), resulting in the voltage spike that you have described. As the pack becomes more balanced, the other cells get a higher voltage and the one cell gets a lower voltage, so then you can charge the entire pack to a higher voltage before the one cell causes the whole pack to block charge.

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mattguy avatar image mattguy commented ·
Thanks for the input and for clarifying what m @thanar meant. I'll keep an eye on the logs from solar, it might be there's an unbalanced cell!


I actually linked the manufacturers to this these so hopefully I hear some good insight from them too. I'd be happy to sacrifice 5-10% capacity to extend the batter life!


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mattguy avatar image
mattguy answered ·

Update: The mppt is set with adaptive absorption so it doesn't enter absorption multiple times of day for long periods. The mppt is set for 14.4v, which is higher than that multiplus so as to try and top off the cells as quick as possible, within reason.


It did no experience the high voltage spike, though I did see 14.9 briefly (see video). The video shows bulk ending with the spike, then a quick absorption and then to float.


I've lowered the multiplus to 14v absorption and limited current to 50amp vs 90.

Any issues with mppt being set at a 14.4 absorption with a potential of the multiplus also charging from the grid, but at a lower voltage? (14v)?


Thanks all for your help




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