Basic questions on li-ion batteries

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akis_T

Newly Enlightened
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Apr 27, 2014
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Hello everyone

I have a device that I will power with 6S li-ion batteries and it draws 1.2A-1.5A constantly. The aim is to maximise the on-time before the batteries run dry.

I have bought and am experimenting with two Turnigy 3000mAh 40-50C "high discharge" batteries.

My idea is to stop at around 3.5V per cell. Fully charged they are over 4.1V per cell.

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My first question is regarding the ""high discharge" bit, do I really get anything by buying the 40-50C batteries vs their 20C equivalents? Remember my load is 0.5C at most.

My second question is about maximising the number of recharge cycles. Is it true that if I stop at 3.5V I will get more cycles out of the batteries? And will that account for the loss of "on-time" had I decided to drain them down to say, 3.2V ?

Finally, I have looked at those Battery Management modules as sold on ebay and elsewhere, these resemble the boards used on laptop batteries, in other words the laptop (or the charger) only provides a constant charging voltage, and the module does the balancing and everything else, so you do not need a dedicated stand alone charger. Has anyone got any experience with these? http://www.ebay.co.uk/itm/221809115255?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

Many thanks
 
In regards to the battery pack. Are you using Li-ion or Li-Po packs?

What I remember from the prime days of my RC was that the LiPo packs for racing were a bit of a compromise. The 20C and 40-50C packs were both generally capable of the same mAh storage per space, but because of the chemistry allowing for faster discharge the 40-50C higher current packs got fewer total cycles out of them. Now chemistries have probably changed a bit in the past 5-8 years but the number of cycles was like 200 versus 500.

For the current draw of 0.5C on those packs I would go with the lower 20C rated packs if you can get more usable cycles out of the battery.

Second answer is a bit trickier considering the low load level. With higher powered RC stuff (we're talking 2000W peaks on a 1/8 scale truggy here) I would set my LVC on the speed controller to around 3.3-3.4 volts. This would give a resting voltage of about 3.6-3.7 generally once the cells were depleted for the run.

Since your use is such a low draw I would tend to think a LVC of 3.5 volts is a good place to start. Your loaded voltage will not differ much from unloaded one.
 
They are Li-Po.

The 40-50C ones are heavier than their 20C counterparts, but not by much. They are also double the price, almost. If for my uses the 20C are just as good or better, then that's all that matters of course. I was wondering whether the 40-50C ones were made with better chemicals and materials and "naturally" could achieve better charge cycles as well as longevity.
 
It would be nice if that were the case, but for this example it just doesn't work that way. They we designed for more demanding loads at the expense of other things.
 
akis_T said:
They are Li-Po.

The 40-50C ones are heavier than their 20C counterparts, but not by much. They are also double the price, almost. If for my uses the 20C are just as good or better, then that's all that matters of course. I was wondering whether the 40-50C ones were made with better chemicals and materials and "naturally" could achieve better charge cycles as well as longevity.
Click to expand...

You would need a custom balancing board with custom shut off voltages built. The board will monitor both total voltage as well as the individual cells.

Mountain electronics should be able to fix you up.

What are you attempting to power at 22 volts?
 
I am thinking of using the ebay board http://www.ebay.co.uk/itm/221809115255?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

Simply to allow me to charge it using a simple 25V supply rather than a dedicated charger.

I got the idea from my laptop batter, and I presume most other laptop batteries. The laptop does not contain a dedicated battery charger, it is the tiny in size module onboard the battery that does the charging/balancing.

Then I will also build a custom arduino board to monitor current draw and individual cell voltages and to disconnect the load when any error conditions occur.

My device is powered by two 24V sources, currently through a mains adapter that provides +/-23V (two independent and floating sources of 23V). With the battery pack I will be able to use the device on the move. For simplicity I want the battery pack to be a simple box and to charge it I will plug in the existing mains adapter lead rather than having to connect a dedicated charger. All I will need to do is probably to increase the mains adapter voltage to a bit over 25V. Then it will be like a laptop, you plug in the mains adapter lead whenever you are near a mains, else it uses the battery. When the adapter cable is plugged in the battery charges through its onboard balancing module.
 
I would still need to charge this battery - I would need a separate charger which is what I am trying to avoid.
 
It's like a TENS machine which runs off a mains adapter but with a battery you could also move around as long as the battery lasts.
 
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