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Battery help ?

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tron87

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I have a project that has very small space allocated for a battery pack im thinking of using two nimh pp3 8.4v batteries in parallel to get the desired capacity is this a good idea for periods of time ranging in weeks ?
 
The batteries probably have 400mAh each so 800 / 5 = 160 hrs and 800 / 23 = 34.8 hrs.
Now those are ideal numbers, in reality it'll be less. Best case is 6 days. Worse yet is most NiMH will self discharge even without a load.

What is this project for?
 
Energizer's old Ni-MH battery was 7.2V (6 cells) at 150mAh.
Their new one is hard to find and is 8.4V (7 cells) at 175mAh.
The new one's voltage gradually falls to 7V in 10 hours when the current is 17.5mA.
 
Weather station+radio transmitter, whats not classified as a low drain device in nimh terms ?

If i parallel two pp3 nimh how can prevent voltage differences between the two forming and possible over discharging one of the pp3?



Take a 175mah nimh pp3 battery for example at any instance how much maximum current can i draw from it without it being damaged though short circuit current ?
 
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I used paralled Ni-Cad cells for years and they worked well. I also have laptop and cell phone Li-Ion batteries with cells directly connected in parallel. Their voltages were very close and stabilized to the lowest of them. Two 8.4V Ni-MH batteries in parallel will make about 9.5V when freshly charged then no current flows between them. When discharging, the voltage quickly drops to 8.4V then slowly drops to 7V. If you discharge below 7V then you risk that a weak cell will discharge past 0V and be charged with reverse polarity by the remaining working cells which is bad.
 
Diode isolate the two battery packs to prevent self discharge. It will cost you 0.7 volts at the input terminal of your device. Why not connect a solar array to recharge the batteries whenever the sun comes out?
 
Diode isolate the two battery packs to prevent self discharge. It will cost you 0.7 volts at the input terminal of your device.
Diodes are not needed since the voltage will stabilize at the voltage of the battery with the lowest voltage then there will be no more discharge between batteries. If one battery is +9.1V and the other battery is +9.0V then their paralleled voltage will become +9.0V.
 
Sorry Batman but, this is one of those times when theory and reality diverge. One battery stack will always be at a slightly higher voltage than the other and will attempt to recharge the lower one. In the process it lowers its own voltage and raises the others as expected. However, it overshoots by a little and the process reverses. If you don’t diode isolate the two parallel stacks, they will self discharge faster than they will if isolated.

If you don’t believe me, try it. Get a couple of rechargeable batteries and charge them to the same point and note how long they take to discharge. Recharge them and connect them in parallel and monitor the discharge time again. To be thorough, recharge again and monitor the discharge in isolation again to verify that the batteries’ discharge characteristic has not changed from the first test run.
 
If you don’t believe me, try it. Get a couple of rechargeable batteries and charge them to the same point and note how long they take to discharge. Recharge them and connect them in parallel and monitor the discharge time again. To be thorough, recharge again and monitor the discharge in isolation again to verify that the batteries’ discharge characteristic has not changed from the first test run.
I connected Ni-Cad batteries in parallel for my beach sound system many years ago. It worked perfectly for a very long time.
Today there are forums where guys connect small Li-Po cells in parallel to power their electric RC airplanes instead of buying new bigger cells and they work perfectly.
I have some Li-Ion battery packs from cell phones and they have cells welded directly in parallel. They work perfectly.

I have a paralleled cells cell phone Li-Ion battery in my hand now. It measures 3.76V with no load. It was last charged about 6 years ago. It powers my airplane motor with a huge propeller perfectly today.
Diodes are not needed.
 
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