Old laptop. Replacement NiCad battery pack is around $50 and puts out 4.8V/1800mAh. I already have several AA-size NiMH batteries putting out 1.2V/1800mAh each. If I put four of them in a $3 case I’m assuming I get the same power at a lower weight and can keep another set charged for quick replacement.
Is that assumption based in fact?
I can’t provide too much useful information, but I’m pretty sure that voltage is cumulative. At least my dad the retired electrical engineer told me that 4 1.5 volt batteries equal a 6 volt battery.
Yeah that’s a pretty safe assumption. Fully charged, your 4 cell pack is gonna be closer to 6V but the working value is 4.8V.
Conversely, if you wired them in parrallel, you have a 1.2V battery at 7,200 mAh
Yes, that is ideally correct.
But laptops are sensitive in their power requirements. With imperfections, resistance between batteries and such, I don’t think they would provide the same amount. I’m not sure what that would do to a laptop, but I wouldn’t risk it.
You’re dad is only telling you half the story. Wiring the batteries in series will increase voltage cumulatively, but wiring them in paralell does not. Parallel wiring has the effect of increase amperage, while series wiring does not.
Good point, UncleBeer, and series was the assumption in our conversation.
Yes, wiring the batteries in series will add their voltages, but provide the same current as one of the batteries.
It will also place all the batteries’ internal resistances in series, which may be a factor, or at least different from the internal resistance of the original-equipment battery. (The higher the internal resistance, the lower the output voltage at a given output current. The internal resistance is placed in series with the resistance of the load.)
How much current does your load draw?
Not only do the voltages add when you connect the cells in series, but so does the total energy capacity. That is, four 1800 mAh cells connected together (in series or parallel) give 7200 mAh. If your numbers are correct you should be able to go four times as long between charges with your home-made battery pack than with the OEM pack.
BTW, when you connect cells or batteries in parallel the internal impedance decreases. The current delivered to a typical circuit will barely change - the current will only go up appreciably if the circuit has a very low impedance.
Hell if I know. Can’t find a manual with that much info.
Sensitive? It’s a Tandy 1100FD with a 10mHz V20 (8088 clone). “Sensitive” is unlikely, especially since it was made in the early 90s, before the great, reliable, and accurate modern batteries. It was probably designed to be comfortable with variable voltages. I also suspect, based on experience with other laptops of that vintage, that the original battery pack consisted of four C-size NiCads soldered together. The packaging I’m planning may add a little resistance but, I suspect, not much.
As to the risk, I got the computer for free. If my plan works, I’m ahead with a lightweight, instant-start machine. If it doesn’t, no big deal.
NiMH batteries are 1.2 volts, not 1.5, so four in series will be 4.8 volts.
Replacing what’s probably a heavy and bulky pack of NiCad battery with four AA batteries sounds like a good plan. And, when the cells eventually go bad, they’re readily replaced with a trip to the drugstore.
The only issue I can see might be with how the laptop charges its battery - you may need to conjure up a way to either disable its internal charging circuit or alter its current output, but I think NiCad and NiMH batteries can be safely charged with the same charger.
I’d expect that as well. There shouldn’t be any appreciable difference in resistance between soldered together and the spring clips in a battery holder. You’re looking at maybe an amp and a half current flow here.
A dirty little battery secret - I don’t know if it’s still true, but a lot of “C” and “D” size rechargeable batteries were just “AA” cells in a larger sleeve, so in a way, you’re being historically accurate.
How do you figure?
Four 1.2V 1800mAh cells connected in series is theoretically equivalent to a 4.8V 1800mAh cell.
Four 1.2V 1800mAh cells connected in parallel is theoretically equivalent to a 1.2V 7200mAh cell.
There is no way to connect four 1.2V 1800mAh cells together to be equivalent to a 4.8V 7200mAh cell without some magic.
Except when I was takling to my dad, we were talking about standard 1.5 volt AAs.
That’s why I will not leave it charging while I’m at work until I know it won’t burn down the house. My rule of thumb, first with engineering but expanded to run my life, is, “It was successful if nothing caught fire it. It was even more successful if it did what I hoped it would.”
Needless to say, I’m not much of an engineer.
Maybe, you need to check the current output of the batteries. I suspect that the AAs won’t be able to put out the necessary current.
Or at least not for very long.
You can arrange batteries in series and parallel to increase the voltage and amperage. You could take 4 of those 1.2V 1800 mA batteries and put them in series to get 4.8V 1800 mA. Then make another identical series and tie them together in parallel to get 4.8V 3600 mA. It doesn’t look like you need to do that, but I figured I’d mention it. And remember that you can’t have too much amperage in a battery. The device will only draw what it needs.
They aren’t 1800 mA batteries, they are 1800 mAh batteries. mAs is a measure of current, and mAhs is a measure of stored energy. Big difference.
Excuse me, mAh is stored current, not energy. Still, big difference between the two.
To be pedantic:
-mAh is a unit of charge. Current is the rate of change of charge, and can’t be stored.
-The OP is asking about assembling a battery from cells. The battery the OP is attempting to replace is most likely made of a similar combination of cells. If the vernacular didn’t fail to make a distinction between batteries and cells, the OP probably would have already known this.
To the posts suggesting putting the cells in parallel: This is a bad idea. NiMH cells (NiCads also) have very low internal resistance, they don’t tolerate parallel operation. If you need higher capacity, use larger cells.