AA vs 4/3A Ni-Mh Batteries - Page 1
Forum section: electronics / beginners

author: MGaddict
author_rank: Contributor
timestamp: February 05, 2025, 04:47:40 pm
content: Aside from capacity, am I going to have issues/start a fire replacing 4/3A Ni-Mh batteries with AA size batteries?I have rebuilt battery packs for laptops before without issues. I simply crack open the battery pack, remove all the corrosion, then replace the batteries in it with new ones and wire it exactly the same way. I recently acquired a 486 Toshiba Satellite Pro which powers on just fine with the battery pack installed and plugged in. It won't power on, however, without the battery pack installed which won't hold a charge at all. I opened it up, like normal, and the batteries are extremely corroded. The terminals and the temp sensor in the pack appear to be fine. For some stupid reason, 4/3A batteries have apparently skyrocketed in price in the last few years and it would cost me about $60 for the batteries, and I can't find any with the exact same 2600mAh to boot. That being said, I have about a dozen AA size Ni-Mh with 2000mAh rating. Obviously they are physically smaller and would probably only power the laptop for a short period of time but I don't really care about running this thing off batteries, I just need to be able to turn on the laptop. What I'm afraid of, is the laptop is going to try to overcharge the batteries which would be bad news. If I had studied electrical engineering in school instead of cyber security, I would probably not have to ask stupid questions like this and probably be able to figure out how to wire the pack so it held no batteries and would still turn on just by having the empty pack installed; that would actually be ideal.Any recommendations?

author: inse
author_rank: Super Contributor
timestamp: February 05, 2025, 05:18:18 pm
content: I don’t think this could cause any problems.If the original battery pack was made from NiMH cells, it should be fine with new ones.Either the charge circuit has implemented the -dV algorithm for charge termination or simple terminal voltage limitation, both should also work with smaller cells.

author: benj38
author_rank: Frequent Contributor
timestamp: February 05, 2025, 05:34:41 pm
content: A few points to keep in mind:NiMH batteries are extremely stable compared to lithium batteries, they almost never ever catch on fire. They very rarely do explode, in the sense of violently releasing gas pressure, if abused and have a faulty release valve.To paraphrase what @inse said, the mechanism built into the charger to detect a full state of charge is not dependent on the capacity of the battery in the sense that it will detect a fully charged 2800mAh battery, but not a fully charged 2000mAh one.If you use a lower capacity battery the rate of charge will be higher. Usually, manufacturers specify rate of charge as some fixed percentage of the capacity rating (e.g., C/2, which means 1400mA for a 2800mAh battery, but only 1000mA for a 2000mAh battery). Would this be too fast? Maybe not, but laptops tend to fast-charge and thus approach the upper safe limit, so please measure the cells temperature during the charge process to make sure they do not get too hot.

author: J-R
author_rank: Super Contributor
timestamp: February 06, 2025, 09:50:49 am
content: How many cells do you need for the pack? These prices don't seem too bad and you could ask for a quantity discount:https://www.ebay.com/itm/335543030830?var=545031775611

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 06, 2025, 02:11:34 pm
content: Quote from: J-R on February 06, 2025, 09:50:49 amI think the OP would need at least 10 of them and thus the price is close to the $60. However, if I were teh OP I would just use the right batteries as they will fit the case better.

author: MGaddict
author_rank: Contributor
timestamp: February 08, 2025, 03:09:23 pm
content: I already have over a dozen AA size batteries. That was the point of the post; would it be okay if I used those? I think I got my answer, so I went ahead an rebuilt the pack. It appears to work just fine. I used some cardboard to fill in the empty space, and the pack only lasts about 45 minutes, but that's perfectly acceptable. It's not like I'm planning on using this laptop as my daily driver.

author: MGaddict
author_rank: Contributor
timestamp: February 08, 2025, 03:18:20 pm
content: Quote from: J-R on February 06, 2025, 09:50:49 amThose really aren't bad at all. Still around $45 for 10 + shipping. I kind of wish I had found that before I rebuilt the pack the other day using the AA. I saved the seller though, I have another laptop I want to rebuild the pack on. Thank you for the link.