Huge voltage drop after a couple minutes

JAY · Post by **JAY** » Thu Jan 26, 2006 9:40 pm

I have a pair of 7.2V, 3300mAh Nimh battery packs that I got from powerizer, with each connected to its own switch, so that I can run the system on just one of the batteries, or both at the same time. After charging both batteries (they'd read about 7.5V), I'd give the system a run, and everything works fine for about two minutes, and then the screen gets "whitewashed", as if the brightness keeps being increased. After that, the screen will go off. I unhooked my batteries from the system and checked their voltage, and both were around 6.4V (which would explain why the screen went off). After about 20 minutes, I re-checked the batteies, and they went back up to about 7.0V (one read 6.8, the other 7.3) on their own.

Could anyone explain what's going on with my batteries? The screen's unmodded, but with the batteries in parallel rating about 6600mAh, I wouldn't think that there would be much problem with supplying enough current to my setup. Any help would be appeciated.

Post by **marshallh** » Thu Jan 26, 2006 9:42 pm

1. You might have a bad set of batteries.
2. The packs may simply be crappy packs. Most won't lose voltage quite that fast.
3. Batteries will always go back up in voltage when not under load.

I'd bet you aren't charging your batteries all the way up. Most cheap chargers are only made for 1500mAh.

What charger are you using?

JAY · Post by **JAY** » Thu Jan 26, 2006 9:54 pm

I'm using the smart charger from powerizer that automatically turns off when the battery is fully charged (with charge rate at 1.8A). Although the ones on the site are now 2.4-7.2V, and 8.4-12V, mine says it's good for 7.2-12V.

When you say the batteries will go back up when not on load, would it take a while for them to do it? I checked my batteries about a minute after I took 'em off of the system and they were still reading that low.

Edit: I should mention that I charge the batteries individually, rather than trying to do them while in parallel.

superdeformed · Post by **superdeformed** » Thu Jan 26, 2006 10:22 pm

Have you tried running it with just a single battery pack? Using batteries in parallel usually doesn't work very well. It's unlikely that the current draw will be the same across both of them so they will discharge unevenly, which will cause more problems (one battery trying to charge the other). If you really want to have them in parallel, at the very least you should have a diode across them (you would obviously want one with a low forward voltage drop, and a reverse breakdown voltage higher than 7.2V). Ideally if you were using them in parallel, you would want each battery going to it's own regulator, and then connect the outputs from that in parallel.

Also, did the battery packs heat up significantly over the 2 minutes the screen was running for? You should measure the current coming off of the batteries as well as check for shorts.

JAY · Post by **JAY** » Thu Jan 26, 2006 10:50 pm

Yeah, I've tried running the system on each of the batteries by itself, and the same thing happens. I didn't notice the batteries heat up when they were running in parallel, and I had them sitting on my lap, so I'd presume it wasn't a big deal.

The reason why I put them on different switches was because I knew that one of the batteries would try to charge the other if the voltages weren't almost identical, so my initial plan was to have the system playing on one battery, and when the screen started to get flakey, I'd switch the other battery on and turn the former one off. I had the same thing in mind when charging the batteries, so I have a charging jack for each battery pack.

I'm starting to think that I have some crap batteries, since the video starts to go bad even when I have the PSone screen off and am running the system through my AV jacks onto my TV.

Super Mulletman · Post by **Super Mulletman** » Thu Jan 26, 2006 11:13 pm

Yes, I know that battery. For RC cars, right? I was hoping otherwise, but if that is the case, I suppose the voltage drops because it's supposed to. I mean, for RC cars, they run a lot, stop, and run some more. Maybe they're not designed to run for so long at once.

superdeformed · Post by **superdeformed** » Fri Jan 27, 2006 7:41 am

Super Mulletman wrote:Yes, I know that battery. For RC cars, right? I was hoping otherwise, but if that is the case, I suppose the voltage drops because it's supposed to. I mean, for RC cars, they run a lot, stop, and run some more. Maybe they're not designed to run for so long at once.

RC car batteries are designed for huge current draws, which is the reason they work well for portables. The voltage is not supposed to drop simply because it's running constantly. If it is dropping that much over a short period of time, then it is losing most of it's charge too quickly, or not fully charged to begin with.

JAY · Post by **JAY** » Fri Jan 27, 2006 9:19 pm

Well, how could you check to see if the batteries are fully charged, other than just reading the voltage drop? My charger automatically cuts the power when the batteries are completely charged, so I wouldn't think that that would be the problem. But if the charger is somehow not doing its job, I should make sure that it's the batteries that I have a problem with, and not the charger.

Anyways, does someone else know of a place where I can buy some reliable batteries? I think I should go with some 8.4V packs instead, but the problem is that I already designed the case with the 7.2s in mind, so I might have to dismantle the pack to stick the 7th battery in a place where it could fit, or just buy another 7.2 pack and wire a single cell to it. Since the screen takes 7.5, I'm a bit uneasy that the picture will start to degrade after I've been playing it for awhile if I'm using the 7.2 battery.

Also, can you use the PSone screen wall adapter to power the unit from an outlet? I don't have it with me right now, but I think that it could output 3A, so I was wondering if I could just use that instead of making my own wall adapter.