Silly question about series batteries

[badr]

TL;DR Summary

Want to know equivalent current.

Hi everyone.

So I am making a Ni-Mh battery , and for space constraints, I need to put 2 batteries in paralell + 1 battery in series with them.

So if I get two batteries of 2100 mah in paralell I get 4200 mah, but if I put them in series with one 2100 mah battery , what would be the equivalent current flowing.

I hope this is clear !

 

[berkeman]

Putting two batteries in parallel gives you twice the current capability at the same output voltage. Putting two batteries in series gives you the same current capability at twice the output voltage. Both configurations double the available energy, as long as you consume that energy efficiently.

BTW, paralleling two batteries is often a bad idea, and you have to know what you are doing when you attempt it. What is your background in working with batteries like this?

 

[hutchphd]

TL;DR Summary: Want to know equivalent current.

So I am making a Ni-Mh battery , and for space constraints, I need to put 2 batteries in paralell + 1 battery in series with them.

I agree with @berkeman that this isually bad practice. Just use 2 cells in series (what does the third actually do for you?)
Shortly before my departure from the company, I once spent most of an afternoon trying to convince my idiot boss that no, one could not get more energy out of a batttery if the cells were put in parallel rather than series.. Most of my bosses were excellent engineers, so this conversation was a shock (bad wordplay) to me..

 

[Rive]

I need to put 2 batteries in paralell + 1 battery in series with them.

The maximal current will depend on the lone series battery, which will get the worst load/usage and will die first.

I would prefer having three in series, with a charger/DC-DC converter circuit to have the right voltage. That would give you the highest current and longest battery life.

 

[hutchphd]

longest battery life.

Yes. This arrangement allows longer use of the basttery at the end of its (charge) lifetime as the terminal voltage declines.

 

[badr]

The maximal current will depend on the lone series battery, which will get the worst load/usage and will die first.

Thanks for the information !

 

[Vitina]

Batteries used in commercial packs are usually matched in capacity for series connected. If one cell depletes first it can be damaged by reverse charging.
Parallel packs are matched for both voltage and capacity. If one cell depletes first it becomes a parasite limiting the voltage in that set.
Temperature is also a factor in parllel packs because it affects voltage of a cell or even within a cell.

 

[badr]

Batteries used in commercial packs are usually matched in capacity for series connected. If one cell depletes first it can be damaged by reverse charging.

Do you mean it can explode ?

 

[sophiecentaur]

Yes. This arrangement allows longer use of the basttery at the end of its (charge) lifetime as the terminal voltage declines.

Series connection is less problematical but there could still be a problem if one of the batteries runs out of useful charge because the are circuits which could produce reverse volts across the duff cell. This would need to be spotted with a voltage alarm / disconnect switch which would be better for the health of the cells.

 

[sysprog1]

The maximal current will depend on the lone series battery, which will get the worst load/usage and will die first.

I would prefer having three in series, with a charger/DC-DC converter circuit to have the right voltage. That would give you the highest current and longest battery life.

Presumably you meant three in parallel. Batteries connected in series produce a voltage equal to the sum of their individual voltages, but the current is not summed, because the series connection does not provide a per-component increase in electrical throughput path capacity, as parallel connection does. Connected in parallel, they produce current equal to the sum of their individual currents.

 

[Rive]

Do you mean it can explode ?

The usual AA Ni-Mh setups are rare to explode, in normal circumstances, with a well set fuse.
There are other types of batteries which might.

Presumably you meant three in parallel.

No. Based on the presented hybrid solution something like 2.4V is needed, but with three battery worth of capacity or with higher current capability than a single battery could produce. If so, three parallel just won't cut it and 3.6 to 2.4 DCDC (tricky, but at least doable) would do better.

 

[badr]

OK i think i better explain it simply .

So if i put one 2100 mah battery in series with one 4200 mah battery . Can this set up result in some sort of equivalent current ?

 

[Rive]

In such setup you can't (well... shouldn't) exceed the maximal current of the lone 2100mA battery.
The available capacity will be not triple, but just ~ double of what a single 2100mA can offer.
Due the lack of balance between the two parts the 2100mA battery will be overdepleted and overcharged in every cycle and will break down fast.

 

[sysprog1]

No. Based on the presented hybrid solution something like 2.4V is needed, but with three battery worth of capacity or with higher current capability than a single battery could produce. If so, three parallel just won't cut it and 3.6 to 2.4 DCDC (tricky, but at least doable) would do better.

The hybrid solution consists of two battery cells connected to each other in parallel, connected in series to a third cell. Assuming 1.5v cells, the voltage for that arrangement would be 3v, and the current available would be that of the third cell.

In such setup you can't (well... shouldn't) exceed the maximal current of the lone 2100mA battery.

I would say can't and shouldn't try -- the two parallel cells are equivalent to a single cell of the same voltage with twice the current capacity, but that doubled capacity isn't available in the series circuit, although it does create a mismatch in the capacities of the 2 resultant components (1 'double cell' and 1 single cell) of the series connection. The voltage of those 2 series-connected components would be twice that of a single cell.

Connecting all 3 cells in series would provide the sum of their voltages, but only the current of 1 of them; connecting all 3 in parallel would provide 3 times the current capacity of 1 cell, but at the same voltage as 1 cell.

So 3 cells connected in series would produce three times the voltage of 1 cell, but no more current than 1 cell, that is, a third of the current of 3 cells in parallel, the voltage of which would be that of 1 cell, but that voltage could be stepped up by DC to DC conversion as needed.

 

[Rive]

Assuming 1.5v cells

Ni-Mh

voltage of which would be that of 1 cell, but that voltage could be stepped up by DC to DC conversion as needed.

Stepping up from 1.2V is not the same as stepping down from 3.6V.

 

[badr]

Ni-Mh


Stepping up from 1.2V is not the same as stepping down from 3.6V.

Good idea , so for example :

I am looking for 9.6 volts , and i only got 14 ni-mh cells of 1.2 volts each.

How can i use a regulator to get maximum current . But not surpassing this pack size :

See this pack , originally got 8 custom size batteries in series but I am trying to fit similar size with 14 AA 1.2 ni-mh Varta cells , and of course with max current.

 

[Rive]

I am looking for 9.6 volts , and i only got 14 ni-mh cells of 1.2 volts each. ... See this pack

That pack is in the size/performance which can explode/catch fire, so the tutorial mode should end here I think.

You may be able to get a pro who can make the pack from the usual 4/3AF NiMh cells and that'll just work perfectly.
Cheap replacements from Ali or such should do the trick too.

You are clearly not at the right level, so the whole DCDC thing is just off the table. Forget it.

 

[badr]

You are clearly not at the right level, so the whole DCDC thing is just off the table. Forget it.

I might be good enough to install a dcdc converter , but the issue is within the charge limiter of the portable device , we don't know if it could still detect the regulator output as a battery and charge it back up.

 

[berkeman]

but the issue is within the charge limiter of the portable device , we don't know if it could still detect the regulator output as a battery and charge it back up.

I don't know if you've already said in this thread, but what is the end application? What are you trying to power with these batteries and power supply?

I agree that you are in the realm of potentially dangerous battery failure -- even if you know a lot about what you are doing, it can still be dangerous. I've managed to have some close calls with battery circuits, including having one catch fire...

https://www.xxlmag.com/hoverboards-are-catching-on-fire/

 

[badr]

I don't know if you've already said in this thread, but what is the end application? What are you trying to power with these batteries and power supply?

This battery is for powering this megohmmeter :

A new battery costs a lot.

 

[berkeman]

So that product comes with an internal rechargeable battery and charging circuit, but you want to convert it to use an external rechargeable battery with your own recharging circuit?

 

[DaveE]

This battery is for powering this megohmmeter :

A new battery costs a lot.
View attachment 344835

Why would you be so concerned with "maximum current" for a device like that? It shouldn't draw much current, I think.

 

[badr]

So that product comes with an internal rechargeable battery and charging circuit, but you want to convert it to use an external rechargeable battery with your own recharging circuit?

Yes , also trying to get the right voltage 9.6 v

 

[Rive]

trying to get the right voltage 9.6 v

A small USB powerbank, with an adjustable DCDC converter?
Even charging it within the device can be solved with some DIY tweaking.

I don't know the current draw so 5V 1A might not be enough, but I would try to avoid messing with batteries if possible.

If it's batteries then I would try to get some professional to replace the cells in a spent pack.

 

[DaveE]

Maybe two identical 5V USB power banks* connected in series to get 10V. That should be close enough. Then try not to completely discharge them in use for best lifetime.

*Newer USB standards allow charging at higher voltage. Make sure you get one that is only 5V.

 

[Flyboy]

I’m with Rive… let the experts handle any repair/overhaul, especially since it’s intended for use on a (hopefully) calibrated test instrument.

 

[badr]

Hi guys I am back again.

I told my father that there is a risk of overheating. But a guy told him it will work though

I am technically not convinced , and I take no responsibility if something goes wrong.

So I am out of this anyways.

 

[sophiecentaur]

We can't argue with "a guy", can we?

 

[badr]

We can't argue with "a guy", can we?

We cant argue with 2 guys :D

 

[berkeman]

So I am out of this anyways.

Okie dokie, thread is done. Tell the "guy" he can cover the damages from the fire.