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I have two 12 V 110 Ah Eco Tree (LiFeP04) batteries wired in series with both batteries having the following technical specification:

  • Built-in BMS: Yes
  • Nominal voltage: 12.8 V
  • Nominal capacity: 110 Ah
  • Energy: 1408 Wh
  • Charging voltage: 14.4 - 14.6

The batteries are then connected to an Iconica IC-BM2000-24 hybrid power inverter (Manual). This in turn is connected to Canadian Solar 410 W High Power Mono PERC HiKU solar panel.

My Iconica hybrid power inverter has the following settings configured:

  • (05) Battery type: User Defined
  • (26) Bulk charging voltage (C.V voltage): 28.2
  • (27) Floating charging voltage: 27.0
  • (30) Battery equalization: Disabled

Both the manufacturer of the batteries and the inverter were comfortable with the use of lithium-ion batteries being used and advised I disable the equalization mode.

Once the sun rises the hybrid power inverter starts charging the batteries. Checking the Bluetooth app for the batteries shows that one battery (A) gets fully charged while the other (B) stops charging at 62%. Despite battery B not being fully charged my hybrid power inverter believes both batteries are fully charged and stops the charging process.

Battery B stuck at 62% is capable of being fully charged if I charge it using a 12V LiFePO4 mains battery charger. Once it's fully charged battery B only charges until battery A becomes full. This continues until battery B goes below 62%, in which case while the Remaining Charge on battery A no longer increases battery's B Remaining Charge continues to increase until the SOC hits 62%.

Currently, the Bluetooth app for the batteries shows the following:

  • Battery A
    • SOC: 100%
    • SOH: 100%
    • Volts: 13.51 V
    • RM/FCC: 109.97 / 110.0 Ah
  • Battery B
    • SOC: 62%
    • SOH: 100%
    • Volts: 13.37 V
    • RM/FCC: 68.11 / 110.0 Ah

Have you any idea how I can get my hybrid power inverter to fully charge both my batteries?

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  • \$\begingroup\$ You ignored 2. CAUTION --To reduce risk of injury, charge only deep-cycle lead acid type rechargeable batteries. \$\endgroup\$
    – greybeard
    May 18 at 11:51
  • \$\begingroup\$ I checked with the manufacturer of the inverter and the batteries and they both said it would work with lithium-ion batteries. \$\endgroup\$
    – A Dark Divided Gem
    May 18 at 12:03
  • \$\begingroup\$ Add that to the question, as well as: Starting from an equal State of Charge (SoC) and operated in series, does the reported state show up? How soon? \$\endgroup\$
    – greybeard
    May 18 at 12:10
  • \$\begingroup\$ What’s the terminal voltage across each battery? How are you balancing them when in regular use? \$\endgroup\$
    – winny
    May 18 at 12:44
  • \$\begingroup\$ I have updated the question to include details from the battery's Bluetooth app but I don't know what you mean by balancing them when in regular use. \$\endgroup\$
    – A Dark Divided Gem
    May 18 at 12:59

2 Answers 2

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A BMS will disconnect the battery charge path if an overcharge threat is recognized. Since your batteries are connected in series, the second battery is disconnected as well and cannot complete the charge cycle.

It is bad style to rely on this safety disconnect feaure of the BMS as a regular end of charge control except the rare cases where the battery documentation allows this use case.

So a kind of balancing is required. You can fully charge both batteries individually before connecting them to the system. This will work for a while, but over time their charge states will drift apart.

At least a passive balancer is required to compensate this capacity drift. These balancer circuits add a resistive load in parallel to a nearly full charged battery to delay the end of charge and give the other batteries in the chain time to catch up with the full one(s). In such a system a safety turn off by a BMS will not occur because the end of charge can be recognized by the power inverter.

It is also possible to charge the batteries, say once a month, individually with a separate charger.

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The BMS in battery number A will inhibit current from flowing when state of charge nears 100%. This will prevent battery A from overcharging (good) and battery B from fully charging.

During discharge the opposite will will happen. So this will need constant monitoring.

The BMS systems are working as intended. Had you not bought batteries with internal BMS you would not be asking this question, the battery would have vented. Or worse, it could have burned down.

Possible solutions I can come up with are, in order of best to worst:

  • Buy a inverter intended for this battery chemistry.
    It will have cell balancing.
  • Buy a 24V battery, if LiFePO4 then again with internal BMS.
    The internal BMS will handle inter-cell balancing, as it does now within the units.
  • Add separate 24V balancing unit.
    There are standard 24V lead-acid balancing units, used in mobility scooters for example. Victron also sells one. A unit like this will keep the units balanced bypassing a limited current during charging/discharging. It won't however correct the unbalance you have now. This needs to be done initially using other means. I cannot predict interoperability with BMS embedded in your batteries, these units are not intended for this chemistry.
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