Li ion battery in series

[kitchsink]

Hi.
I'm new to DIY lights and I want to build a set of bike lights using Cree Q5 leds at as low a cost as I can.I want to use a 14.8v li ion battery,so can I wire 2 x 7.4v (4000mAh) batterys in series (because its cheaper than one 14.8v battery) when they both have a built in PCB?

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2456

If so,can I just then get a 14.8v charger(or does the PCB's affect this also?).Any recomdations of same would be appreciated

Thanks,
Dermot

 

[DM51]

Welcome to CPF, kitchsink.

Your question covers various aspects of what you want to achieve, but the principle part relates to batteries, so I'll move it to the appropriate section.

 

[kitchsink]

 

[Mr Happy]

All right, I will try and respond to your bump

First of all, lithium ion cells come with a nominal voltage of 3.7 V, so a 7.4 V battery is actually two cells in series. And 14.8 V is four cells in series.

Now one of the critical factors of Li-ion cells is the maximum voltage they can be charged to. They generally must not be charged above about 4.2 V or bad things start to happen. What this means is that four in series must not be charged up beyond 4.2 x 4 = 16.8 V.

However, this is where things get fun. For instance,

16.8 = 4.2 + 4.2 + 4.2 + 4.2

but also:

16.8 = 4.1 + 4.1 + 4.5 + 4.1

Do you see the problem? If the charger is only looking at the total voltage, it does not know whether any individual cell is being charged beyond its maximum voltage.

The answer to this problem is a balancing system that evens out the voltage between all of the cells during charging. This balancing system might be built into a battery pack, or it might be an external device, or it might be built into the charger.

Whatever happens however, you do need to be concerned about this. You must make sure you take care of balancing (which means extra wires and "balancing taps"), or you must charge each cell individually or in parallel.

Look at the sub-forum above, "Smoke and Fire", to understand how bad it can be if you overexcite lithium ion cells by improper charging. It is not pretty.

 

[VidPro]

However, this is where things get fun. For instance,

16.8 = 4.2 + 4.2 + 4.2 + 4.2

but also:

16.8 = 4.1 + 4.1 + 4.5 + 4.1

One time i lost $150 bucks because that is exactally what happened, in a large scooter Bank.

in the end it was
0.0 = 4.1 + 4.1 + 0.0 + 4.1

very depressing event , when i thought i had the math worked out, and the batteries didnt USE My Math :devil:
to think that $100 balancing charger (at that time) would have solutioned it, was even more depressing, nowdays you can get a balancing charger for half that.

 

[kitchsink]

Thanks Mr Happy,
I've checked out some stores for Li Ion batteries,but never factored in the high shipping costs!!:thumbsdowJust going to have to do some research into NiMH batteries instead.:thinking:

 

[Chodes]

Thanks Mr Happy,
I've checked out some stores for Li Ion batteries,but never factored in the high shipping costs!!:thumbsdowJust going to have to do some research into NiMH batteries instead.:thinking:

Many forum members buy LiIon cells here. I do.

http://www.cpfmarketplace.com/mp/showthread.php?t=197375
Shipping is $4.50 ( up to six batteries, additional cells $0.50 each ) . Registered mail w/ tracking is $3 extra.

You should read a lot about LiIon cells and chargers before buying and using though. Notice AW sells "black" cells and the IMR orange cells. You will need to search for info on that, too much info for a single post.