2 Lead acid batteries in parallel. uneven discharge - Page 1
Forum section: electronics / general

author: Vincentb
author_rank: Newbie
timestamp: July 08, 2022, 08:41:57 pm
content: Hello everyone.We have 2 lead acid batteries that are wired in parallel. Each of those batteries are 95Ah each, so we have a total capacity of 190Ah. Now if we apply a load of 100 amps on those batteries, one battery discharges with 15A and the other battery discharges with 85A. Does this mean that the one battery with 15A discharge is defective? The batteries are exactly the same size and brand.Any help would be appreciated.

author: nightfire
author_rank: Frequent Contributor
timestamp: July 08, 2022, 08:53:24 pm
content: With this kind of batteries you have to care for several things. Amongst those are:- same age and ageing factors- same resistance in the circuits to the batteries to ensure totally even flowBut when they are discharging being in parallel with a ratio of 1:6, then something is terribly wrong.Reason #1 being one of the cells defective, reason #2 being some joints not being tightened really well.Reason#3 due to slight differences in resistances, one battery was charged a bit better than the other, which leads to a slight difference in voltages internally, so the better battery is also charging partly the weaker one.Discharge/charge them separately and measure, which capacity they are able to handle. If the 15A one is not reaching the nominal capacity anymore, there is the answer...

author: Gyro
author_rank: Super Contributor
timestamp: July 08, 2022, 09:08:12 pm
content: QuoteIf you edit your original post, you can change the thread title to 'parallel' rather than 'series'. It will help to avoid confusion.

author: Vincentb
author_rank: Newbie
timestamp: July 08, 2022, 09:28:58 pm
content: The batteries were origionaly connected in parralel but the fuse poped in between those batteries. So one was more discharged than the other. So i charged the battery to the exact same voltage as the one who is charged by the motorhome. So the voltage difference is close to zero when i replugged the fuse. The battery i charged deliverd the 85A. The one charged by the motorhome only delivers 15A. When i measure the current between those batteries when there is no load, there also is no current. And when you say slightly different voltages internally, that is something i cant measure? And that difference could couse the problems i have? I have them connected now in parallel and let them balance and will remeasure tommorow.

author: nightfire
author_rank: Frequent Contributor
timestamp: July 08, 2022, 09:38:24 pm
content: Popping fuses nearly always have a good reason.I would separate the two batteries, and discharge them after each other, measure the capacity given, then recharge and measure what with identical voltages could be charged. When there is a significant difference, one battery is damaged.

author: ConKbot
author_rank: Super Contributor
timestamp: July 09, 2022, 08:19:52 pm
content: What kind of lead acid batteries are these? If flooded, by all means give them a good charge to a gassing voltage to get them completely full, and balance the cells internally. Check water levels to make sure neither is short on water.Just to be clear, both batteries are the same model number and age?There are so many variations of lead acid batteries that an imbalanced discharge would be expected of differing batteries. I.e. starter vs deep discharge batteries, calcium-lead vs lead-antimony plates. Even agm vs flooded with lead-antimony plates can have differences.

author: JohanH
author_rank: Frequent Contributor
timestamp: July 09, 2022, 08:52:50 pm
content: Lead acid batteries can get oxidized, which causes internal resistance to increase. You can measure this by putting it under load and observe voltage drop. There are also special lead battery capacity meters for this purpose. Some batteries can be "rejuvenated" by a few hard discharge and charge cycles (there are different schools of thought about this). But it can still be hard to get back full original capacity. By its own the battery could still work well for a purpose, but it could cause some imbalance when parallel connected. It's not unknown that a lead battery has some faulty cell, but the only one I've personally have seen was caused by physical damage (battery was shaken hard in a vehicle). As already hinted, charge the battery separately using a high amperage charger (trickle charge won't do it).

author: richard.cs
author_rank: Super Contributor
timestamp: July 12, 2022, 08:54:25 am
content: How exactly is the wiring arranged? Small differences in wiring resistance can result in uneven charge and discharge. Depending exactly where the fuse is its resistance may be significant as well. This issue is explained well here:https://www.impactbattery.com/blog/post/how-to-charge-marine-and-rv-batteries-in-parallelOf course it is still possible that one battery has been damaged by being over-discharged.

author: Berni
author_rank: Super Contributor
timestamp: July 12, 2022, 10:01:47 am
content: Quote from: Vincentb on July 08, 2022, 09:28:58 pmLead acid batteries are difficult to tell how full they are from just the voltage. They have a lot of short term memory of what they have been doing in the last few hours. To really get the two batteries matched up you should connect them in parallel, then charge them for a whole day. Then try discharging them.If they still don't deliver the same current then either one of the batteries is going bad or you have a wiring problem(as others have said above).