# Very simply low power charger

Thread Starter

#### WFC98

Joined Jan 21, 2019
12
Hi there,

I am designing a hand-cranked charging system for a uni project in which I want to charge a small (18mah) NiMH battery.
I want to obtain all of the charge in around 2 minutes of cranking but this would mean that the charging rate would be 20-30C which i'm assuming is WAY too large and would destroy the battery.

My idea is to have a secondary power storage device like a supercapacitor and then trickle the charge into the battery over a longer period of time.
I just wanted some reassurance that this idea is plausible and if anyone has heard of this being done before for some advice of where to look.

Cheers
Sam

#### BobTPH

Joined Jun 5, 2013
3,655
18 mah? Is this a tiny button cell?

Bob

Thread Starter

#### WFC98

Joined Jan 21, 2019
12
Yes, its that sort of range (18-25mah) the type found in hearing aids

#### BobTPH

Joined Jun 5, 2013
3,655
Okay, maybe doable then, let’s do some calculating.

1.2V x 18 mah = 21.6 mWh = 78 Joules (Watt seconds)

A capacitor charged to V volts has 1/2 C V^2 Joules. So, using 3V, it we get:

78 = 1/2 * C * 9 or

C = 2 * 78 / 9 = 17 F

Allowing for the fact that you cannot use all the energy in the capacitor and losses along the way, you will need, maybe twice that or 34 F.

As far as getting 78 Joules in 2 min, that is less than 1W (120 J in 2 min)

So, I think it is doable, even with a fairly inefficient device.

Bob

Thread Starter

#### WFC98

Joined Jan 21, 2019
12
Hi Bob,

Thanks for that, very useful.

So you think the supercapacitor is the way to go?

With a simple rectifier and voltage limiter into the capacitor and a regulator controlling the voltage into the battery, is there anything else i havent thought of.

Cheers
Sam

#### BobTPH

Joined Jun 5, 2013
3,655
Charging NiMH is not simple. You probably should look into a chip that does the correct protocol for it.

Bob

#### Gautam@96

Joined Jan 21, 2019
1
Thread Starter

#### WFC98

Joined Jan 21, 2019
12
Charging NiMH is not simple. You probably should look into a chip that does the correct protocol for it.

Bob
Hi Bob,

Any idea where to start?
I've seen some setups with LM317 systems as well as more complicated chips wich require higher voltages.
This is only a very low power system though?

Thread Starter

#### WFC98

Joined Jan 21, 2019
12
Hi there,

I was wondering if anyone could give me an idea on how to build a very simply charging system for a 25mah (very small) NiMH coin cell battery.

Cheers
Sam

#### ericgibbs

Joined Jan 29, 2010
13,602
hi WFC,
Look at this video, basic charger.
E

#### Alec_t

Joined Sep 17, 2013
12,198
I want to obtain all of the charge in around 2 minutes of cranking but this would mean that the charging rate would be 20-30C which i'm assuming is WAY too large and would destroy the battery.
Your assumption is correct. So the charge time must be much greater than 2 min.

Thread Starter

#### WFC98

Joined Jan 21, 2019
12
I am hoping to charge the battery from a charged capacitor but not sure whether I need to any chips to control the circuit.

I want to keep it simple as this is only a prototype, just needs something that can charge the battery in around 5-10 hours.

#### BobTPH

Joined Jun 5, 2013
3,655
Hi Bob,

Any idea where to start?
I've seen some setups with LM317 systems as well as more complicated chips wich require higher voltages.
This is only a very low power system though?
You can safely trickle charge at C/20 taking a full day to charge. To safely get faster charging, you need to detect end of charge or you will destroy the battery. As I stated, there are ICs that will do this for you. I suspect you can find one that runs on 3V.

Bob

#### wayneh

Joined Sep 9, 2010
17,153
I am hoping to charge the battery from a charged capacitor ....
Why? What are you really trying to do?

If you do the math, you'll see you need an enormous capacitor (or a bank of them) to charge even a small battery. Getting the energy out of the capacitor requires a boost circuit that can keep delivering current to the battery as the capacitor voltage sags. If you have so much capacity that the voltage sag is not a problem, you could get away with a simpler charge controller but this would still be an inefficient system.

Maybe there's a better approach to solving your goal?

Thread Starter

#### WFC98

Joined Jan 21, 2019
12
I am trying to generate the power using a hand crank, so preferrably enough to fill the battery in around 1-2 minutes.

But obvisiously if I blew that power straight into the battery it would be around 20-30C and would heat up.

Thats why the idea was to have a secondary storage device

#### wayneh

Joined Sep 9, 2010
17,153

#### Alec_t

Joined Sep 17, 2013
12,198
the idea was to have a secondary storage device
Use a rechargeable second battery of higher Ah capacity, which can take a higher charge current.

#### wayneh

Joined Sep 9, 2010
17,153
Use a rechargeable second battery of higher Ah capacity, which can take a higher charge current.
It's homework and I assume the battery is a given constraint. No real-world thinking allowed.