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LiFePo4 (123 Smart BMS) + MultiPlus 12/1200/50; absorption settings (not charging)

Just installed my MultiPlus. Inverter and pass-through AC work great, just can't get the settings right for the charger. I'm using the "fixed" charge curve.

- Fixed charge curve stages are "bulk", "absorption" and "float".
- From the shape of the graph I reckon that "bulk" and "absorption" are just different names for CC (constant current) and CV (constant voltage) - terms that are used more often in Lithium (building) communities. (edited)

The absorption voltage - which I can manipulate using ve.config - was initially set to something like 14.xx V. This would cause the charger to remain in Bulk mode for too long, right up to the point where my BMS would cut the input due to hitting the Vmax setting for my cells.


I then changed that setting to 13.68 V - as that sits just above Vbypass and correlates nicely to other - lithium specific - chargers that I have (and this was recommended by the BMS people). The battery is currently depleted to about 38%, voltage sits at 13.2 v (which is nominal voltage). However, when connecting the charger to AC input, it does not start (bulk) charging! As I believe that the absorption voltage setting is correct, it must be a misunderstanding of how these settings are used.

  1. How are these voltage settings used? Are these limits - i.e. the chargers moves to absorption once the voltage gets close to my setting (of 13.68 V), and to float once the voltage hits the float voltage setting?
  2. Based on one: what would be correct settings? 13.68 for absorption (pretty sure about that one) and then a higher value for float?
  3. If I'm completely missing the point, does anyone have a good write-up or explanation about how this should work? I'm counting on the charger being able to move to absorption and later stop charging once certain voltages are hit
  4. What am I doing wrong?!


I'm not using the chargers' relay inputs, since the BMS has it's own relay. What I'm trying to achieve:


  • MultiPlus should bulk charge until the battery voltage hits the absorption voltage setting (13.68 V).
  • MultiPlus should then do absorption charging for the set time period
  • Additionally (ideally), the MultiPlus should stop charging at a certain max voltage. I can't find any setting for that.


MultiPlus Quattro Inverter ChargerMultiplus-IILithium Batterycharger
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Update

After reading more sources on the internet, regarding charging LiFePo4 batteries, I've been trying different voltage settings. The results are somewhat better, but still not good enough.

  1. For the absorption voltage, I tried 14, 14.1, 14.2, 14.3, 14.35 and 14.4 V. I don't want to go higher than 14.4, because that would be damaging for the cells and would trip the BMS protection (> 3.6 VPC).
    1. At 14.4 it charges at 90% of the set charge current until 87% SOC (which is actually 87% of 80% capacity). The battery voltage then stabilises at around 13.4 V. and the MultiPlus goes into "float".
    2. Between 14.0 and 14.4 the charger chargers at various lower currents
  2. I set the float voltage to 13.5 V. Below that the charger would turn off much quicker. That's about 3.375 VPC - which is actually slightly too high
  3. I checked the voltage at the battery wire terminals to see if wire voltage drop caused the problem. This does not seem to be the case: the voltage at the terminals is the same as reported by the BMS.

So - with the new settings (14.4 absorption and 13.5 float) the battery bank charges to about 1.76 kWh (capacity is 2.4 kWh, 80 DOD = 2.0 kWh). Better than before, but not very satisfactory. Also, the charger still goes into "float" which is not necessary for LiFePo4 and may even cause a lower lifespan.

1 Answer
WKirby avatar image

I set my Absorb to 14V (3.5VPC) and hold it there for 1 hour. Any cell balancing that needs to happen is done in this time frame. If you don't want charge up to 3.6VPC then you'll need to have a longer Absorb time for the battery to Absorb more energy, but you might find that the charge current tails off quite quickly in the Absorb stage which means that the battery is not taking in any more energy for that particular Voltage and it will be "full".
I set my Float to 13V (3.25VPC) but you can just make this lower if you don't want to have any charge current flowing in the Float stage. If you make it 12V then it will be 3VPC which is well below the nominal Voltage

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Hi @WKirby,

at 13V, a LiFePo battery is about 20% charged, so I don't understand your float voltage.

Please elaborate......

I mean that if the Float Voltage is set quite low then the battery will stop charging, because the actual battery Voltage will be higher than the set Float Voltage. It's a little trick to stop the battery from charging any further if that's what you're aiming for.

With Lithium batteries, Voltage is a very loose way of determining SOC.