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Old Apr 27, 2004, 9:37 AM   #1
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Default Need advice on choice of batt type

Hi everyone.
I am trying to make my own battery pack for laptop. The reason is this laptop is very old. And it is impossible to find it's battery here in israel.
The original battery is marked 10.8V. It contains 3 units of 3.7v li-ion cells. And it's completely dead. That's the reason i am trying to replace.
So my idea was to put 9 ni-cd or ni-mh 1.2v cells. I have large amount of 1800mah and 2000 AA 1.2v rechargables. So i recharged them and connected consequently. Laptop worked only 10min. Is i researched and read some info on the Internet, The reason is: Ni-mh cells can't stand high load. (0.5-1C) they drop voltage very rapidly.they have much higher internal resistence incomparison to ni-cd's. After laptop shut down (volgate dropped below critical level) cells were almost full. I also have some old aged 800 mah and 900mah ni-cd (not ni-mh!) cells. And i made same expreiment - i connected them (9 cells) consequently - Laptop worked about 30-40min and cells DID hold such high load (~1A). After laptop shut down, ni-cd cells were almost empty. Which means laptop used them to about 90% inspite that ni-mh cells were used for only 10-15%.
The problem with ni-cd's is they have too insufficient capacity. So than i connected 1800mah ni-mh cells in parallel by 2. And took 9 such pairs (each one consists of 2 ni-mh cells connected in parallel ) Total 18 cells. In this time situation was much better. Laptop worked for about 2 hours.(which is OK for me). If i understand correctly, load (discharge current) on each single cell was 2 times lower now (for 0.5A).

So my question to you is the following: i heared many times that connecting ni-mh/ni-cd cells in parallel is not recommeded and will end their life very quickly. Is it true? Can you advice any other way of connecting cells for my case.

P.s. Battery pack size is not important to me. What IS important - so laptop could work from cells enough time.
Thanks in advance.nt
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Old May 2, 2004, 11:09 PM   #2
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Check out Dansdata.com - he's an electronics fanatic and I can recall having come across "battery for old laptops" issues within some of his articles (you'll have to search his site).
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Old May 10, 2004, 7:52 PM   #3
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Hi, perhaps I can help. I used to race RC cars and built my own battery packs. Sub C cells for racing can be had up to about 3000ma in nicad form. A 9 cell pack will get the voltage you need. Or 4400 ma D cells could do as well. Sanyo is the battery of choice by racers. A good peak charger by Tekin can be had on ebay for about $50. if you watch for it (BC112 A). It will charge quickly and with much more punch than a slow charger will. It depends partly on how much you want to spend. I actually have 10 4400 cells I'd be willing to part with...unused, they didn't come cheap. Other peak chargers are out there as well, I can advise you on which ones are good if you like, but hte tekin is a good deal if you can find one on ebay.
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