I have a number of mobility scooters. A few years ago I replaced the stock 2 x 12V Lead Acid batteries with 2 x 12V LiFePO4 batteries. I charge these using one of my two 13A Victron Blue Smart Chargers. In addition each scooter is fitted with a Vicron BMV-712 Smart Monitor.
The standard lead-acid batter charger outputs only 8A, and the inbuilt charging wires were not designed for 13A, so I fitted separate higher capacity wires direct to each scooter’s 24V battery terminals.
This arrangement has worked very well for around 4 years, however recently I noticed that on occasion the normal charging sequence (Bulk-Absorption-Storage) was not being followed. Instead Bulk charging would only last a few seconds then the charger skipped to Absorption and then Storage modes. The scooter wills till be charged up, but it takes longer. And the next day it may work fine. This random misbehaviour occurs on either scooter, and with either of my 13A charger. I have noticed sometimes the voltage drop between the charger and the batteries is a bit high, e.g. 0.5 or 1 volt). Consequently I suspect the charging wires and associated connections have developed higher resistance. Shortly I will pull them apart and check, but wonder if there are better/alternate suggestions for the probable cause(s).
I am also hoping to get a technical explanation describing the purpose and reasoning behind Absorption charging.
For example: I notice when the Bulk 13A charging mode is skipped over and Absoprion starts prematurely, the current drops to around 1.5A at first but soon ramps back up to 13A. This high current only occurs when the charging sequence is abnormal, presmumably the charger detects that the battery is not really charged enough for low amps Absorption mode, so it ramps it up. Perhaps the purpose of Absporption mode is to ensure the battery is still charged even when there is excessive voltage drop across the charging cables?
See attached a screenshot of the charging history from one of my chargers, showing the occasional abnormal charging sequence. But I can’t find a way to attach it here. Suffice to say over 20 days it shows abnormal charging about 5 or 6 times. (At least one scooter is charged each day, and I often travel the same distance each day. So, as shown on the screenshot, the scooter typically requires around 10-13 Ah of recharge).
Sounds like you are doing it right. Double check that you have your charge settings correct. Too high and you could be prematurely wearing down the batteries, too low and they may not be charging all the way. Either one could cause issues. In the Bluesmart you can use advanced settings to get your charging voltages very specific. Let us know what your charging voltages are at and we can advise.
Generally speaking absorption is a lower amperage longer(sustained) charge meant to top off the batteries and balance the cells. One possibility is that the cells are not being balanced with a good absorption charge often enough. This can also be adjusted in the advanced/expert settings.
Another thought is that 4 years of constant charge cycles can add up.
Here’s a pic of the charge profile graph.
You can see the voltage climbs with maximum amperage during bulk.
Then voltage stabilizes and amps slowly decrease as battery reaches full charge(during absorption). This is where it gets the final 10% or so of charging and if it is set correctly forces the cells to balance.
Thanks for the good info. I’ve been using Custom/Expert mode to set 28.8V fully charged (recommended by the battery manufacturer/supplier). I also reduced Absorption time to the minimum available. This used to be 1 hour but a recent Victron update allowed me to drop this to 30 mins. I chose this because I can get 4%+ or even 5% voltage difference between the two 12V batteries very soon after the 28.8V is reached (this may be after only 10mins of Absorption). The manufacturer/supplier was not concerned by this (although I am). I’d be interested in your take … is this risky? One scooter has 105Ah capacity, the other 54Ah. Obviously I only use 1 scooter at a time, (the other being a spare) with typically 15 - 20Ah daily usage. I haven’t noticed any deterioration in performance and the batteries are spec’d for very long life (8000 cycles) so I expect their capacity/range has hardly diminished.
When charging two lithium batteries in series, it is essential to use a balancer. If one BMS disables charging, the voltage can rise significantly. Victron sells a reliable balancer you can find the manual for it online. I have attached it for you. I have also used balancers from Heltec and PowMr, and they all perform very well. The Victron balancer operates passively and bleeds up to 1 amp from the higher voltage battery using a resistor. In contrast, the Heltec and PowMr balancers can transfer up to 5 amps to the lower voltage battery.
You should charge them individually with a 12V charger and see if they are out of balance.
You definitely need balancing. I also think your absorption setting should be set to fixed. I’m not sure adaptive will be giving you good cell balancing. Not sure if the balancers that @owenb79 is talking about would change that or not. Maybe someone else can comment on that.
I’m not totally convinced this is true. I know balancers are important for lead acid since they have no BMS. I don’t know that two 12V LFP batteries need it. Assuming they both have a quality BMS are wired correctly with good size cable of equal lengths, and are being charged properly I don’t see why they wont balance.
OK I’ve ordered a Victron balancer and will test it to see if that improves the situation.
Meanwhile I’m very interested in your last comment @TherealKT and where the discussion goes from here, partly because I may struggle to fit the balancer in the scooter battery bay or nearby. I did pay close attention to wiring size vs max load amps (~120A) including the BMV712 midpoint monitoring, and the batteries certainly have an internal BMS. FYI see attached the specs of the batteries I use (one scooter has a pair of the iTECH054 which are 54Ah, and the other a pair of iTECH120X which are 105Ah).
I’ve also reverted the charger preset to the Victron default Li-ion setting to see if charging to 28.4V instead of 28.8 helps
I also have a 24V Li-ion battery bank for a ‘small’ ~750W solar system that uses a Victron MPPT 100/30 Solar charger controller. This also needs balancing so i can use the Victron balancer there. I prefer to go with reputable brands as up-time and reliability are key for both uses (especially the scooters). I asked Gemini AI about this topic and it explained the difference between passive and active balancers, and that the latter is significantly smaller. The Victron balancer ((BBA000100100) is a passive design = larger so I’m now looking for an active balancer designed for 24V Li-ion battery systems. AFAIK Victron doesn’t sell these ?
