I am planning on a 1 cubic inch robot as my next project in robotics and the biggest challenge I am facing is on the battery. Lithium polymer seems to be the best option in terms of weight and the required current output and rechargeable!

The requirement in circuit is around 400mA with 3.5V, After searching a lot in Internet I could best get to this battery which fits my dimension and voltage requirements.

But its still low on current so I plan on using them in parallel. I read that Li-ion and Lipo batteries are very sensitive and might damage the battery or cause fire if used incorrectly So I need some help on this!

  1. Can I use them in parallel ? Is that the right and safe thing to do ?

  2. If I use them in parallel then can I charge them to in parallel ? or do I need some special balancer circuit ?

  3. Can somebody help me with a decent charger circuit for this cell ?

  4. Anybody knows a better battery than this one that I can use in my application?

  5. Any other points/suggestions that will help me on this project that I missed out ?

  • \$\begingroup\$ Thanks for the link, I found a protection circuit that you may be interested in all-battery.com/… \$\endgroup\$ – jsolarski Feb 15 '11 at 0:26
  • \$\begingroup\$ This might turn up to be useful but here in the link it says "from discharging current more than 14A" but in my case the the max discharge current is only 240mA so shouldn't I have a circuit which prevents it from drawing more than 240mA..? If yes then how can I build/buy one of those ?? \$\endgroup\$ – Zaxx Feb 15 '11 at 4:04
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    \$\begingroup\$ are you sure it isnt 240mAh? and yes that is the protection circuit needed, I am still looking for a protection circuit diagram myself, but from what i can tell its cheaper to buy a pre-built one(not my favorite choice) \$\endgroup\$ – jsolarski Feb 15 '11 at 15:49
  • \$\begingroup\$ Yeah its cut-off is at 14A but I did some with 5A but thats still high.. I need something that matches 240mA. Even I would prefer making one myself If i could get the circuit diagram but couldn't find any so far. \$\endgroup\$ – Zaxx Feb 15 '11 at 16:03
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    \$\begingroup\$ instead of using the 240mAh 1C you should be looking for a higher C rating and mAh, you will get a longer run between charges. some things that you will need to consider are...the runtime of the bot, total current (400mA), so lets say you need to run your bot for 1Hour @ 400mA you would need a 400mAh 1C battery, or a 200mAh 2C for the same hour. but your Battery datasheet should give you a better idea of runtime - reference allaboutcircuits.com/vol_1/chpt_11/3.html \$\endgroup\$ – jsolarski Feb 16 '11 at 5:49

We have used LiPo in parallel successfully. Parallel cells must have the same batch #.

When charging, use a voltage regulated source set to (slightly less than) the 4.2 volt cell charging voltage.

Look at a DC-DC converter with a current limit at 1C (240mA)

So you need a modular DC-DC converter for rated at 400mA, at 4.2V. The upright-PCB ones are available from RS etc...

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  • \$\begingroup\$ Is it safe to put 400mA ?? in any case if the current in one of the battery goes less then the 2nd battery would me getting above its max charge voltage and might damage it ? Or do we need to put some sort of cutt-off circuit to each of these batteries ? \$\endgroup\$ – Zaxx Feb 15 '11 at 3:57
  • \$\begingroup\$ charge in parallel. charger voltage limited to 4.2V. 2nd cell cannot see more than charger voltage. Cells in our application are 8x parallel and work fine. Cell with lowest resistance charges first, comes up to 4.2V and charges no further. \$\endgroup\$ – Tim Williscroft Feb 15 '11 at 21:24
  1. you can use it in parallel, and this does make things more complex. And depending on how long your robot is supposed to run for will decide if its right or wrong for your application

  2. you can charge them in parallel but you will need a 2 cell charging circuit

  3. I wish i could help you with this circuit but, but thats over my head

  4. here is one from spark fun 1000mAh - link this one has the protection included, other places to look would be place that sell micro heli replacement parts

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  • \$\begingroup\$ +1 for a good link to a great battery- was looking for one like that. :) \$\endgroup\$ – Piotr Kula Aug 21 '11 at 20:50
  • \$\begingroup\$ ebay.co.uk/itm/… - 3.7V 2500mah Battery // or ebay.co.uk/itm/… 3.7V - 6aH \$\endgroup\$ – Piotr Kula Aug 22 '11 at 7:12

1) You can use them in parallel without a balancing circuit if this is a one-unit school or hobby project. The worst case scenario is that your robot prototype will burn up or blow up in the lab. If you were building this for mass deployment you might get a custom battery made, as big as you have space for and the right shape, that would have a higher capacity without having to use multiple cells in parallel.

2) You can charge them in parallel without a balancer, but you might want to cut the charging rate down to less than 1C of the whole pack so that an unbalanced cell is only getting 1C for that cell as a worst case. So if you used 2 x 240mA-hr cells, you'd want to charge the battery at 240mA for a 2-hour charge. For 4 cells, the same 240mA gives C/4 for roughly a 4-hour charge.

Extra: you need to make sure the battery is disconnected as a power input to the robot when it gets discharged to the minimum voltage, 2.75V for the example listed. If your robot keeps trying to run until the battery is over-discharged, the battery could be permanently damaged and might have to be replaced. Don't treat it like a capacitor that can be run down to zero volts.

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I would suggest not using the two cells in parrallel. Instead:

  1. Use the two cells in series,
  2. Use a simple buck converter to step the voltage down,
  3. Protect the cells from over-disharge (low voltage),
  4. Use a two cell charge IC for charge control (loads of these available from microchip, TI, Linear Technologies, Intersil, maxim and so on)
  5. Not worry about charge balancing (not essential for a two cell pack - still useful)

Some part numbers of two cell charger IC's to consider: ISL6251, bq24170, LTC1731, MAX1873, MCP73861.

Also the microchip website has a great application note for simple DC/DC (buck) converters using a small PIC - AN216

Going parallel will lead to potential issues in the life of the battery pack - series will be much better

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  • \$\begingroup\$ Just reading this interesting question- i see that the answer for running in paralleled was chose.. but you say that this can have an issue. Even laptop batteries are connected in parallel and then use the buck converter as its the most efficient. Why would you suggest series then? \$\endgroup\$ – Piotr Kula Aug 21 '11 at 20:49
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    \$\begingroup\$ laptop voltage is circa 10-12 V. This is three cells in parrallel. this is double up for standard batteries and tripled up for extended batteries. The primary charging would occur across the three in series with switches to isolate the parallel cells. The reason is each cell is different and will take charge at different rates so forcing the cells in parallel can potentially damage them and reduce \$\endgroup\$ – smashtastic Aug 22 '11 at 6:29
  • \$\begingroup\$ their capacity and lifetime \$\endgroup\$ – smashtastic Aug 22 '11 at 6:29

A buck conveter and multicell battery charger is way to complicated for a school project. I work developing hardware and im telling you, that would take you a long time. If you dont need more than 4 v use as many cells as you want in parallel. Parallel batteries are inherently ballanced. Use a simple dc power supply @4.1v to charge them. Make sure not to discharge them lower than 3.5 so their life time is not affected. Dont charge them higher than 4.1v, and dont leave them charging once they reached 4.1v If you're designing a robot ease your time in mechanical and software improvements. Good luck

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  • \$\begingroup\$ Thank you for your answer! Please, try to edit it to improve formatting of units (capitalization and spacing) and separate the various points, to better cover poster's problems. Moreover, try to address the answer to a generic user and not only Zaxx (in a wiki-style) \$\endgroup\$ – clabacchio Nov 13 '12 at 20:23

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