Battery charging in series - Page 1
Forum section: electronics / beginners

author: hermitengineer
author_rank: Regular Contributor
timestamp: September 07, 2021, 08:35:37 am
content: I have a solar inverter setup that uses lead-acid batteries for storage. The inverter is 48V input, and the charge controller has 48V charge capability.When I looked at it last weekend, the series voltage was below 48V and the inverter was at its dropout voltage. I looked at the individual batteries, and 3 of them were 12.5v but the 4th was 10.5v. Charge current was shown as 1A but it was also blinking to indicate that it detected a problem with the batteries. I removed the 10.5v battery, and the controller downshifted to 36V and charged up the remaining 3 batteries without issue.So, I did some research, but it was inconclusive. Some sites claim that series-charging will lead to uneven charge between the batteries, and suggest battery balancers. Other sites say that the charged batteries will just pass the charge current through. Okay, so who's right?On one hand, a lead-acid battery is internally a bunch of separate batteries connected in series. So if it's bad to series-charge, then it's always bad to charge a lead-acid battery. On the other hand, battery banks such as in laptops will split the batteries so that they can be charged in parallel but used in series. So it would seem that they follow the "series is bad" train of thought.Any authoritative sources to clear up the mud?

author: Whales
author_rank: Super Contributor
timestamp: September 07, 2021, 09:47:28 am
content: Lead acids tolerate overcharging up to a certain current. Do your batteries have model numbers on them? The most authoritative place would be their datasheets, if they exist. They may have a "trickle charge max current" or something similar.Cells within one battery are well matched, so the amount of imbalance between cells during charging is usually minimal (unless a cell had died, at which point the whole battery is mostly useless anyway). As long as a final Vmax is also set on the charger: most of the over-charging of individual cells will be done at a lower current anyway.Different sizes/brands/batches of lead acid batteries in series are not really "well matched", so if they are all given the same current then they will all charge at different rates. Trickle charging is fine (the smaller batteries will shrug the extra off) but higher charging currents might be a problem (?)

author: figurativelythedevil
author_rank: Contributor
timestamp: September 07, 2021, 03:35:11 pm
content: Charging lead acid batteries in series will inherently lead to mismatches over time. Generally what you're supposed to do is monitor the lowest cell voltage with a battery monitoring system, the lowest cell being referred to as the pilot cell. Depending on how you use your batteries, you will periodically have to perform an equalization charge, the procedure for which should be laid out in the battery manufacturer's data sheet. Also be aware that your float voltage for a 48v nominal battery system should be around 54V, so if you're only charging to 48V, you'll never fully charge.

author: hermitengineer
author_rank: Regular Contributor
timestamp: September 08, 2021, 12:17:05 am
content: Thanks for the replies.And yes, the charge controller is aware of the types of batteries. It can do multiple kinds of lead-acid, and I think it even does LIPO. You have to set the battery type when you connect it. So for the 4x12V batteries it will try to keep them at whichever voltage is needed for the battery type (in my case, flooded deep cycle), and that seems to be somewhere around 54V.Do you think a battery balancer would help to equalize the charge? For reference, it's a device that claims to monitor the battery voltages individually and shunt around the better charged batteries in order to keep them all at the same voltage while charging.

author: figurativelythedevil
author_rank: Contributor
timestamp: September 08, 2021, 01:30:46 am
content: I'm not exactly sure of the efficacy of those sorts of things. Personally I would recommend periodically doing an equalization charge. Your charger might actually have that functionality built in, or allow you to do one manually based on the battery manufacturer's specifications. I would definitely explore that first before spending money on another piece of gear, especially as equalizations are standard practice.

author: tautech
author_rank: Super Contributor
timestamp: September 08, 2021, 05:43:40 am
content: Quote from: figurativelythedevil on September 08, 2021, 01:30:46 amYep this ^.Any decent solar charge controller has battery equalization function and either auto and/or manual.Depending of the cell type equalization will be performed at an appropriate level for the battery chemistry.For the battery at 10.5V a good charge should be undertaken then let it sit for a few hours and monitor the OC voltage for a couple of days. If it falls away to under some 12V it's toast for any serious work.

author: mikerj
author_rank: Super Contributor
timestamp: September 08, 2021, 08:17:10 am
content: Sounds very much like you have a dead cell in the 10.5v battery. Lead acid cells tolerate charging in series very well without special balancing circuits since they can accept overcharging with negligible issues provided it's at a low rate and not prolonged.

author: strawberry
author_rank: Super Contributor
timestamp: September 09, 2021, 07:05:32 am
content: aren't PB batteries self balancing, atleast wet.12V 6cells adding more in series wont change, except bad/weak/leaky battery can get charged in reverse polarity during discharge (positive terminal turns negative(trash))

author: fordem
author_rank: Regular Contributor
timestamp: September 09, 2021, 02:56:11 pm
content: Quote from: hermitengineer on September 07, 2021, 08:35:37 amSeries charging of lead acid batteries has been done, as they say, "since time immemorial" - it will eventually lead to some degree of uneven charge (state of charge) and that is dealt with by "equalizing the batteries", which is done by a "mild overcharge" - this allows the undercharged cells to reach full charge and during the process there will be some amount of overcharge on the other cells, which generally will not cause a problem - the batteries will gas, and may need to be topped up (if they are wet cells)I'll just mention in passing that my background is/was UPS power and among the installations I've supported were 80KVA units with as many as three parallel strings of 40 series connected 12V 120Ah VRLA batteries - the battery cabinets had no electronics, just batteries, cables & circuit breakers, no form of "balancing" was provided, nor did any appear to be necessary.QuoteI don't think I've ever seen a laptop with a lead acid battery, NiCd, NiMh & some form of lithium - the charge/discharge requirements are completely different.