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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 05-13-2013, 10:23 PM   #1
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Default parallel charging, batteries at different voltages.

I do a lot of parallel charging. The basic rule of parallel charging is you can charge together any packs (different c rating, different mAh capacity) as long as they are the same cell count and approximately the same voltage when you hook them together.

The 'rule' about being the same voltage can be a pain at times because it means in theory you cant hook together a half charged and a depleted pack due to the fear of high amps flowing between the two batteries. The rule of thumb I'd heard was that the packs needed to be within 0.2v of each other.

I got to thinking if this 'rule' was overly conservative. From flying yesterday i has several 3s 2200mAh packs, most discharged down to 11.1v and one only discharged to 11.5V. The 'rule' says that I cant couple these together for charging due to the voltage difference being too great. I figured that I could hook them together through a wattmeter with the high charged battery on the input and the low one on the output and see what current flowed.

So to cut a long story short I hooked the 11.5 and the 11.1V battery together and momentarily got 6A but this settled to 3.5A within a minute. These batteries are 2200mAh 5c charge rated which works out at 11A, so no problem at all for the battery.

conclusion is that about 0.5V difference between batteries shouldnt be an issue for modern LiPos with 3c or greater charge rates, so can stop checking all my batteries before I hook them together

Now i got brave.. I was wondering what would be the worst that could happen if i connected a fully charged 12.6v Lipo up to a discharged 11.1v lipo, thats 1.5v difference

And the answer was... Briefly 23A flowed, but this dropped very quickly and within 30 seconds the current was within the 11A charge rate. I left the batteries connected for several minutes until their voltages balanced. At no time did either battery get the slightest bit warm or did any 'puffing' occur.
While I'd not recommend connecting fully charged and fully depleted batteries together based on my experiment nothing dangerous occurs and no short term battery damage is evident.

One thing you really dont want to do though is connect batteries of different cell counts. I've done this twice by mistake and it's not pretty
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Old 05-13-2013, 11:27 PM   #2
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That's nice because after similar experimenting a while ago I concluded that 0.5V was close enough for most purposes. In fact it doesn't even seem to matter if they're high charge rate batteries or not because the older low rate ones tend to have higher IRs so the inrush current is correspondingly lower.

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Old 05-14-2013, 12:43 AM   #3
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Originally Posted by JetPlaneFlyer View Post
I do a lot of parallel charging. The basic rule of parallel charging is you can charge together any packs (different c rating, different mAh capacity) as long as they are the same cell count and approximately the same voltage when you hook them together.

The 'rule' about being the same voltage can be a pain at times because it means in theory you cant hook together a half charged and a depleted pack due to the fear of high amps flowing between the two batteries. The rule of thumb I'd heard was that the packs needed to be within 0.2v of each other.
For the folks in the USA, you can also measure this "Circulating Current" with one of those AC and DC clamp on ammeters. Sears Roebuck sells their Craftsman #82369 meter for under $60. This meter can measure DC currents in a 0-40 and 0-400 Amp ranges. The accuracy is 3% which is good enough for 95% of our electric model needs.

Note that most clamp on ammeters are AC only, and not useful for our DC powered electric models.

http://www.sears.com/shc/s/p_10153_1..._tnt=39869:4:0

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Old 05-14-2013, 06:45 AM   #4
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id be worried about melting battery wires or over charging/discharging dissimilar packs.

I used to recover some tiny 3 cell 800 mah battery by using a jst 12v wall charger. hooked a battery checker to the balance port. this worked well for a while, but the #1 cell started to overcharged when near full capacity. I believe it was a 300 mah charger. once I got all cells over 3 v's id hook it right to the balancing charger and all was well.

id just be worried that the balancing"parallel" charger would over volt a single cell and ruin or light up the battery.
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Old 05-14-2013, 08:46 AM   #5
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No need to worry about over charging. The cells are all connected together so the voltages of all batteries and of all cells balances out. They can only ever charge to the same voltage, as governed by the charger cut-off setting.

Excluding connecting different cell counts, my experiment showed that melting wires isn’t a possibility. Even if you accidently connect a full and a discharged battery together you will not get near the batteries max discharge rate (though you may well exceed the charge rate of the 'flat' battery). When I did just this neither the batteries nor wires even got warm.

The one thing that you do need to be careful about is that all batteries are the same cell count. My recommendation on voltage would be that all batteries are within half a volt of each other. Use a balance charging board that allows the balance leads of all batteries to be connected. Leave the batteries hooked together on the board for a minute or two for voltages to equalise before starting charging, especially if charging at high ‘c’ rate.

Aside from that you can hook together any LiPo batteries and parallel charge them. The max charge rate is calculated as the 'c' charge rate (of the lowest charge rated battery), multiplied by the total Ah capacity for all batteries added together.
If you have lots of batteries then parallel charging really is the way to go, it saves LOADS of time. To make the most of it you need a good quality charger with high amp output capability.

I can balance charge twelve 2200mAh 3s batteries simultaneously in about 15 minutes using my new iCharger 4010b. That means I don’t have to spend hours charging individual batteries and I don’t have to plan hours or even a days in advance of going flying. If the weather looks good I just put the batteries on charge and by the time I’ve put the planes in the car they are done. It’s hard to overstate what a difference this makes.

It’s also better for the batteries because I don’t need to pre-charge and store them fully charged.
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Old 05-14-2013, 01:32 PM   #6
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I was wondering how different the voltages could be - good to know.
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Old 05-15-2013, 12:52 PM   #7
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Originally Posted by JetPlaneFlyer View Post
I do a lot of parallel charging. The basic rule of parallel charging is you can charge together any packs (different c rating, different mAh capacity) as long as they are the same cell count and approximately the same voltage when you hook them together.

The 'rule' about being the same voltage can be a pain at times because it means in theory you cant hook together a half charged and a depleted pack due to the fear of high amps flowing between the two batteries. The rule of thumb I'd heard was that the packs needed to be within 0.2v of each other.

I got to thinking if this 'rule' was overly conservative. From flying yesterday i has several 3s 2200mAh packs, most discharged down to 11.1v and one only discharged to 11.5V. The 'rule' says that I cant couple these together for charging due to the voltage difference being too great. I figured that I could hook them together through a wattmeter with the high charged battery on the input and the low one on the output and see what current flowed.

So to cut a long story short I hooked the 11.5 and the 11.1V battery together and momentarily got 6A but this settled to 3.5A within a minute. These batteries are 2200mAh 5c charge rated which works out at 11A, so no problem at all for the battery.

conclusion is that about 0.5V difference between batteries shouldnt be an issue for modern LiPos with 3c or greater charge rates, so can stop checking all my batteries before I hook them together

Now i got brave.. I was wondering what would be the worst that could happen if i connected a fully charged 12.6v Lipo up to a discharged 11.1v lipo, thats 1.5v difference

And the answer was... Briefly 23A flowed, but this dropped very quickly and within 30 seconds the current was within the 11A charge rate. I left the batteries connected for several minutes until their voltages balanced. At no time did either battery get the slightest bit warm or did any 'puffing' occur.
While I'd not recommend connecting fully charged and fully depleted batteries together based on my experiment nothing dangerous occurs and no short term battery damage is evident.

One thing you really dont want to do though is connect batteries of different cell counts. I've done this twice by mistake and it's not pretty
Originally Posted by slipstick View Post
That's nice because after similar experimenting a while ago I concluded that 0.5V was close enough for most purposes. In fact it doesn't even seem to matter if they're high charge rate batteries or not because the older low rate ones tend to have higher IRs so the inrush current is correspondingly lower.

Steve
This corresponds very closely to tests I ran several years ago with 10C-15C first gen packs. At the time 1C was considered the max safe charge rate. I was very comfortable connecting packs with up to a 1 volt difference.

As a result I made up a few multiple parallel charging harnesses and separate but similar parallel harnesses for the balancing taps. One set for 3S, one for 4S etc. That way a charger with a single port - all that was available back then - could charge and balance any number of packs. Of course, chargers were relatively low power for the most part so charge times were longer.

They come in real handy after a long period of no flying when you want to charge several packs to get ready for a days flying. I can charge and balance up to 6ea 3S packs at once and dont have to worry about starting voltages. With the newer hi power chargers you can even do it quickly

I think I need a signature.
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Old 05-15-2013, 12:57 PM   #8
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By the way, if its a concern, you can lower the inrush current when connecting several packs by connecting the packs with the lowest voltage together first. Then add the packs with higher voltages progressively.

You can also lower the difference if you use parallel balancing harnesses like I do. Connect the balance taps together first. The taps have a higher resistance and the average current will be somewhat less.

I think I need a signature.
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