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Old 10-07-07, 06:19 AM   #1
diff_lock2
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NiMH Voltage drop.

I made a pretty cheap (30eu) 14.4v pack out of AAs. I run a 20w Bulb with that pack, that means the current should be below 2A (discharge).

Heres the problem, with out any load the pack measures 15-15.5v (18 right off the charger). Then i hook up a 20w halogen and it drops to 12-12.5v!! This pretty much is counter productive to overvolting!

If i hook up a 8w lamp i get 1v drop to around 14v, thats what i expected on the 20w.

What actual voltages are your 20w+ systems running?
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Old 10-07-07, 11:56 AM   #2
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What is the mAh on the AAs? A 20W halogen will pull 1.67A at 12V, 2A at 14.4V. If your batteries are at or below 2000mAh, you might just be putting too high of a load on them. This seems to be the case, since a 8W lamp (0.8A at 14V) doesn't cause such a big voltage drop.
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Old 10-07-07, 12:37 PM   #3
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Originally Posted by wattly View Post
If your batteries are at or below 2000mAh, you might just be putting too high of a load on them. This seems to be the case, since a 8W lamp (0.8A at 14V) doesn't cause such a big voltage drop.
My thoughts as well. Best bet is to take a look at the spec for the batteries to determine max current draw
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Old 10-07-07, 12:56 PM   #4
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Here is the embarrassing part... They are rated at 3400mAh! MY *****!

I was suckered in to buying these cells cause i though well they are at least 2500mAh... but no. I have only charged the pack as a whole once and the cells individually twice. I am hoping for better performance after a few more charges (as a whole pack)

I didn't buy from here... but they are the same.
https://www.vimacdirect.nl/multishop...908bacf094c0c1
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Old 10-07-07, 02:09 PM   #5
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3400mAh should be able to handle a single 20W light, even overvolted. It is recommended that you cycle the pack 4-5 times for optimal performance, so that might just be the issue.
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Old 10-07-07, 02:32 PM   #6
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When you mean cycle you mean total discharge then fully charged? cause i have not let the pack fully discharge, i don't what the cells you reverse polarity (the weaker ones).

Have have balanced the pack though with a c/10 charge.
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Old 10-07-07, 09:51 PM   #7
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Well, here is the instructions I got with my NiMH battery pack.

1. Charge the battery pack fully before using
2. Discharge the battery pack fully (down to 1.0V per cell) before charging it again or till it won't operate your device
3. Repeat steps 1&2 four times to condition the battery so it can reach it's full capacity

I noticed a big improvement from the first time I used it compared to how it works now after the conditioning.
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Old 10-09-07, 02:27 PM   #8
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I think i just got ripped off, thats all. They are 2300mAh cells rated at 3400. Oh well i think i will have to stick to the hour run times.
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Old 10-09-07, 02:56 PM   #9
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Forgive me if I've got the wrong end of the stick here, but 2300mAh has nothing to do with how much current you can pull from them.

Internal resistance and cell-electrode chemistry defines that.
Cells specifically designed for very high current drains/low voltage drops tend to have lower mAh capacities due large electrode area.

2400mAh etc. means you could draw perhaps 600mA for 4 hours. 2400mA probably for less than 40mins.
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Old 10-09-07, 07:32 PM   #10
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WTF!? 3400mAh AA's? Now that's the highest claim I've seen out there. To the best of my knowledge and I've been looking into batteries and such for about the last 7 years but currently 2700mAh is the highest in the AA class and the good cells are Sanyo 2700mAh which have been independantly tested to be ~2750mAh. You do get what you pay for when it comes down to cells. Yes Sanyos are expensive but you get what you pay for when it comes to batteries.

There are too many companies out there with inflated mAh claims. I have never ordered from this site but many independant testers order from this company and they have a good reputation on thier products and after sales support.

This is a little dated but the guys probably busy to update it. This is the site of the battery tests and some cells don't even come close to thier claims.
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Old 10-09-07, 08:55 PM   #11
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Forgive me if I've got the wrong end of the stick here, but 2300mAh has nothing to do with how much current you can pull from them.
Right, the Ah rating doesn't directly determine the current pull. However, most of the stuff I've seen (and, admittedly, I'm not expert on this stuff), says that you want to pull less amps than the battery has Ah. So, pulling 2A on a 2000mAh battery is too high of a load. I'm sure it varies from battery to battery, but that's just what I've seen bandied about.
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