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Fruitfly01

LiPo Storage and Voltage Percentage.

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Hi guys, hope you can help.  

I'm just doing a little LiPo maintenance.  I haven't used my cars for a while so the batteries have been sitting in storage.  Today I wanted to check on their voltages and do a quick storage/balance charge if required, to keep things equal in the packs.  I'm not sure how much the voltages drift over time, or how long it takes till voltage goes below 3V a cell. 

So I bought out two chargers that have a storage function and one of those battery checker/lcd thingies.

Charger 1 = Overlander RC6-VSP - Storage down to 3.7v per cell (7.4v -2s)

Charger 2 = HTRC HT100 - Storgae down to 3.8v per cell (7.6v -2s)

Batteries as numbered

Core RC number 1 4000mah 20/40c

1s =3.737v

2s = 3.747v

Total = 7.477

percentage charged 14%

Core RC number 3 4000mah 20/40c

1s = 3.751v

2s = 3.755v

Total = 7.506

percentage charged 14%

Core RC number 4 4000mah 20/40c

1s = 3.927v

2s = 3.921v

Total 7.848v 

percentage charged 64%

Yuki LiPo 4000mah 30c

1s = 3.693v

2s = 3.696v

Total = 7.389v

Percentage charged = 12%

NVision 4500mah 45c

1s = 3.834v

2s = 3.848v

Total = 7.682v

percantage charged 46%

 

Sorry to waffle on a bit, but I wanted to give you guys all the info I had in front of me.

What set alarm bells off in my head was the discrepancies in the percentage of charge in each battery.  Of course those with a higher voltage right now have a higher percentage of charge, but it just seems like a big jump up from those batteries with a lower voltage inside and percentage of charge.  A point 4 voltage difference increases the current capacity up to an extra 50%.  Seems odd to me.  But I don't know electrics and too well, and my maths is appalling.  

Am I missing something?  And what is the better storage charge percentage in relation to storage charge voltage?

No batteries were harmed in the writing of this waffle.

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I personally wouldn't compare each battery to each other because of manufacturer tolerances, etc. Cheaper batteries may not last as long, etc. The key is voltage per cell. Storage is ideally at 3.85 but can be slightly lower and slightly higher with no real issues. If you use storage charge mode on your battery, it is usually 3.85V and should balance. Although a lot of people say that storage charge is 50% capacity, it is really slightly less. For those that are extremely low (below 3.7 or close to it), bring them up slowly (low amps). For those really really low, using Nimh mode helps too.

For me, I don't concentrate on getting my storage to be exact unless I'm using the charger. I typically try to run my battery to storage mode instead so as a result the final storage voltage will vary a bit. No big deal for me.

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Thanks for the info.

I'm not comparing batteries to each other.  I know they'll all have differences.  I've had batteries balance in no time, and others take an age.  My main query is to do with end voltages in percentage of charge in storage mode.

Like I say, the Overlander balances at 3.7v a cell, so 7.4v for a 2s battery for storage.  On the battery checker, it's telling me the battery is at 14% of capacity.

The HTRC charges each cell to no less than 3.8v a cell, so 7.6v for a 2s battery for storage.

This extra point 1 volt per cell charge equates to  50% (give or take) of capacity, according to the checker.

That seems like a huge difference in percentage for a point 1 volt increase of power.

None of the batteries are broken - none have ever gone below 3v a cell, as per the rules.  The batteries are good.  I'm just questioning the numbers.

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I'd question your checkers way of working out percentages more than anything. Proper battery meters for (real) cars usually dump some current over a ballasted load to see how much the voltage drops and work out capacity from there. 

Your best bet for testing is to just use them. Capacities will change over time, different makers will be more or less economical with the truth, different tolerances on manufacturing may mean different cells may be marked the same but have a slightly different capacity. At the end of the day the only thing that makes any difference is how much play time you get per pack. 

Everything will turn out ok.

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What does this mean?

"Like I say, the Overlander balances at 3.7v a cell, so 7.4v for a 2s battery for storage"

3.7 is pretty low for storage charge and to "balance" simply means to get both cells to be the same voltage. How a charge does that is regulate each cells separately. So if one cell is at 3.85 but the other is at 3.80, the juice going to 3.85 is stopped but the juice is continuing (albeit at a lower rate) to the cell that reads 3.80. You can balance at any voltage, in theory. I'd set your storage voltage higher.

Like the above post, I question your checker. To me, the important thing is voltage, not %. Assuming there is some level of accuracy to that.

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On 5/29/2020 at 5:40 PM, howaboutme said:

What does this mean?

"Like I say, the Overlander balances at 3.7v a cell, so 7.4v for a 2s battery for storage"

 

That means that Overlander charger, in storage charge mode, will charge a 2 cell battery to 7.4v, balancing the cells to 3.7v each.

There's nothing wrong with the cell checker, that I can see.  The HTRC charger also has an independent cell monitor and balancer, and gives the same readings as the checker.

I'm going to run a few more tests and see what I find.

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I found this chart on the web.  It seems to confirm the numbers I'm seeing on my equipment with regards to percentage and volts per cell.  

49955020201_c5123cfa21_o.jpg

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The voltage across a cell and the charge driven into it by the charger are extremely non-linear.  This is the reason that so much more charge is contained within the cell for such a small voltage increase.  You are on an almost horizontal part of the curve.

With regards to storage charge, I hear some sources saying 50% of total charge is ideal, others 50% of total energy stored.  Those are quite different, as each mAh of charge at the high end of the pack's voltage is carrying a lot more Joules of energy than each mAh when the pack is almost dead.

  • Thanks 1

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That makes sense, and confirms what I've seen.  Thanks.  

Although none of the packs have been damaged when stored at 3.7v a cell (14%), I think I'll er on the side of caution and keep them at 3.85v (50%) a cell from now on.

Thanks

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