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Making battery pack, need help with LED battery indicator

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kloan

New Member
Hi, I just found this forum. From the looks of other threads I'm sure someone here can help me..

I'm making 2 kinds of battery packs, one with 4 AA and one with 8 AA. I would like each of them to have 4 LEDs to indicate how much juice is left in the batteries. I've scoured the web, but haven't been able to come up with any plans.. so if someone could help me out that'd be great!

Thanks :)

P.S. I'm kinda newb with electronics..
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: Making battery pack, need help with LED battery indicato

kloan said:
Hi, I just found this forum. From the looks of other threads I'm sure someone here can help me..

I'm making 2 kinds of battery packs, one with 4 AA and one with 8 AA. I would like each of them to have 4 LEDs to indicated how much juice is left in the batteries. I've scoured the web, but haven't been able to come up with an plans.. so if someone could help me out that'd be great!

You could use an LM3914 LED meter to indicate the battery voltage, but this only gives a rough indication of battery condition.
 

k7elp60

Active Member
Making a battery pack

Nigel is right. The major draw back of teh LM3914 is the voltage divider
reistors are not very precise. I have a circuit I build for testing 12V lead acid batteries that could be possiblly adapted to your battery packs. Some important information is the type of batteries you have. If they are nicad or nicklemetal hydride the characteristics are fairly close as to full charge voltage and cutoff voltage. If they are alkaline then the full charge voltage and the cutoff voltage is different. I would be happy to share the schematic with you and give some ideas on what changes to make.
 

kloan

New Member
Well, I was hoping that I could have a choice between Alkaline and Nimh. I guess since they are different voltages that it'd make things complicated.. I really don't know too much about electronics... I think I might just keep it simple and stick to just the batteries.. :)
 

Nigel Goodwin

Super Moderator
Most Helpful Member
kloan said:
Well, I was hoping that I could have a choice between Alkaline and Nimh. I guess since they are different voltages that it'd make things complicated.. I really don't know too much about electronics... I think I might just keep it simple and stick to just the batteries.. :)

Yes, they are different, 4 alkaline will gvie you 6V, 4 NiMh only gives you 4.8V - I used to have an old Kenwood 2M FM portable transceiver, it came with battery holders for 10 NiCd rechargeable batteries (to give 12V), you could also use non-rechargeables, and they provided two dummy batteries (just shorting links) to keep the supply at 12V.
 

kloan

New Member
I got this schematic from someone on headwize.com, which has the use of zeners, and I'm wondering how it would connect to the positive and negative wires from from the battery pack. The battery pack was going to connect to pins 1 and 2 on a firewire port, so where abouts does this fit? Oh, a pushbutton is going in there too.. also, I'm sticking with 8 AA now.
 

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audioguru

Well-Known Member
Most Helpful Member
Re: LED battery indicator

The graph of the voltage of a discharging rechargeable battery shows that the voltage barely changes between 85% charge and 15% charge. Today I was at work using my incandescent (with filiament) flashlight with 4 new,
NI-MH AA cells and I used it for about 2 hours when the brightness (and therefore voltage) remained the same, then all of a sudden it got dimmer and quit. The dimming lasted for only 5 minutes. That shows me that you cannot measure remaining charge capacity by measuring the voltage using an LM3914 nor zeners. 2 of the cells were completely discharged and the other 2 still had a lot of charge remaining. The 4 cells came in the same package and were recently charged at the same time so they must have differing capacities. Reading the threads about battery chargers will show how complicated it will be to ensure that a cell or more will not be ruined by reversal during discharge, or all cells will not be ruined by overcharge.
 

kloan

New Member
What if the batteries I am using though are completely charged, or what about if I just use Alkaline?
 
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