Different Capacity Battery Series Capacity - Page 1
Forum section: electronics / beginners

author: sammyhwang
author_rank: Contributor
timestamp: December 26, 2013, 11:41:19 am
content: I will be using li-ion batteries in my next project. Was wondering if I had the following setup...3.7V 5000mAh (in series with) 3.7v 1200mAhWhat would the capacity be? Also do I charge it as if it is a 7.4v xxmAh battery? or special charging?Sorry if the question is really simple, just want to knowThanks!

author: Psi
author_rank: Super Contributor
timestamp: December 26, 2013, 11:46:28 am
content: Capacity will be 1200mAh and the extra capacity of the other battery is wasted (never used).This is because, when discharging lithium batteries in series you have to stop when one cell gets to min volts.That will happen to the 1200mAh cell when you've taken 1200mAh out of it.At that point you've also taken 1200mAh from the 5000mAh cell, so it has lots of capacity remaining, but you cant use it without damaging the 1200mAh cell.Connecting them in parallel solves this issue

author: amyk
author_rank: Super Contributor
timestamp: December 26, 2013, 11:47:58 am
content: Not more than 1200mAh, because trying to pull any more charge out of the smaller one at that point will just reverse its polarity and damage it. In series configuration you must use cells of the same capacity and keep them at the same charge level to make use of their full capacity, since current flows through both of them. Google "cell balancing".On the other hand, connecting them in parallel (make sure their voltages are exactly the same first!) will make them behave like a single cell of 6200mAh.

author: fcb
author_rank: Super Contributor
timestamp: December 26, 2013, 12:12:41 pm
content: As others have pointed out, 1200mA.Depending on what you are driving, efficiency etc.. you could boost(convert) the 5000mA pack to 7.4v (or whatever you actually need), you'll get something like 2000-2500mA capacity from a smaller size.

author: Zero999
author_rank: Super Contributor
timestamp: December 26, 2013, 01:36:44 pm
content: It's a bad idea. When the lower capacity cell is discharge, there will be lots of charge still in the higher capacity cell which will try to charge the other cell in the reverse direction, causing smoke and fire.When connecting cells in series, always use the same capacity, manufacturer and age cells.

author: sammyhwang
author_rank: Contributor
timestamp: December 26, 2013, 01:44:26 pm
content: Thanks. I think I will use:3.7v 5000maH series with 3.7v 5000maHThe above in parallel with3.7v 5000maH series with 3.7v 5000maHThis shoud give me 7.4v at 10AH right? So do I charge it as if it is a 7.4v 10AH battery? Or is isolated special charging required?

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 26, 2013, 01:51:56 pm
content: Woah woah woah woah. Hold on a moment.Putting batteries in parallel gives combined current delivery.Putting batteries in series gives combined voltage.Putting two cells in parallel will give you 3.7v 10ahPutting two cells in series will give you 7.4v 5ahPutting two cells in parallel, and then, put another two in parallel, to form 2 lots of 3.7v 10ah.Then you put those two parallel packs in series to give you 7.4v 10 ahRelating to the posts prior to your most recent post;As others have stated, you are limited by the capacity of the lower cell. However, you are not limited to 1200mah, you are limited to less, for a number of reasons;Difference in internal resistance: the smaller battery will likely have a higher internal resistance, leading to lower capacity at higher loads.The cell will lose capacity with time and with charge cycles.The smaller cell will be more influenced by heat/cold.

author: IanB
author_rank: Super Contributor
timestamp: December 26, 2013, 04:02:57 pm
content: Quote from: sammyhwang on December 26, 2013, 01:44:26 pmAs I think Peter Mitchell told you (not quite sure because his post was word salad), you should connect up your batteries not like that, but like this:Put two 3.7 V 5000 mAh batteries in parallel. Put two more 3.7 V 5000 mAh batteries in parallel. Now you have two 3.7 V 10 Ah batteries. Put these two in series to make a 7.4 V 10 Ah battery.To charge your battery use a suitable dedicated lithium ion charger. It must have low voltage detection, high voltage detection, CC/CV charging control, and preferably a balancing function to keep the two batteries properly balanced and equally charged.Don't try to make a lithium ion battery or charger unless you know what you are doing. It is best to buy one ready made. A 10 Ah battery will make a big fire if you get something wrong.

author: tszaboo
author_rank: Super Contributor
timestamp: December 26, 2013, 04:45:41 pm
content: Quote from: sammyhwang on December 26, 2013, 11:41:19 amLi-ion manufacturers when they make battery packs, use less that 1% matching between multiple parameters. Putting the two in series is a very bad idea. Charge and discharge characteristics are really different for the two, say, you charge your pack 5 times and one of the battery will be 3,5V the other will be 3,9V until you kill the whole pack.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 26, 2013, 05:28:05 pm
content: Quote from: IanB on December 26, 2013, 04:02:57 pmHey, I'm trying my best, trying to be social(and socially acceptable) over the christmas break really is cutting into my sleeping time. But yes, word salad indeed.

author: IanB
author_rank: Super Contributor
timestamp: December 26, 2013, 06:59:33 pm
content: Quote from: peter.mitchell on December 26, 2013, 05:28:05 pmI guess I missed a smiley (sorry about that)

author: eKretz
author_rank: Frequent Contributor
timestamp: December 26, 2013, 10:51:37 pm
content: I have a related question. Say you disassembled a battery pack from a laptop...these should all be properly matched cells, shouldn't they? If the cells are connected in series, how can they be individually monitored for voltage? Would it be okay to make a 3-cell pack from some of these batteries removed from a matched pack and have them charge properly in series? Could one theoretically expect the pack to charge ok and the batteries to remain fairly balanced, or would it be better to use a battery holder and only charge them in parallel?

author: IanB
author_rank: Super Contributor
timestamp: December 26, 2013, 11:09:37 pm
content: There is an electronic module inside a laptop battery pack. I am not sure, but I imagine it may include a balancing function. It could monitor each cell for over voltage and discharge it if necessary.

author: digsys
author_rank: Unknown
timestamp: December 26, 2013, 11:31:17 pm
content: Quote from: eKretzGenerally, once you balance a matched set, and don't abuse them, they can stay "matched" for years. There are other methods that "good"BM controllers use to determine if an individual cell is failing. Temperature sensors, bulk energy in and out, rate of charge profiles.In our race packs, I have individual monitoring, but our packs are pushed to the limit, and SAFETY is vital.The answer then is - YES, they would very likely stay matched and as long as you make sure you don't push them past their top andbottom voltage limits, OR over-current and over-temperature, then you're good to go.Actually, placing them all in parallel the FIRST time, and running a couple full cycles, is one of the conditioning methods we use,but you shouldn't need to bother with that.For peace of mind, check each cell voltage. They should match within mV to 10s of mV MAX. DON'T check them fully charged or closeto discharged though.