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Li-ion Battery Discharge Calculation
Li-ion Battery Discharge Calculation
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Old 16th January 2013, 12:56 PM   #1
teleman is offline teleman  Norway
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Default Li-ion Battery Discharge Calculation

I am attempting to power a small circuit drawing around 100mA max with 6, 3100mA li-ion batteries in series providing 22.2v. The batteries will be connected to a pcm (protection circuit board) through which the circuit will draw the supply. The PCM board can be adjusted for max discharge current.

Should I set the PCM board for 0.1C or 0.05C max. discharge? ( 310mA & 155mA respectively) Is there any significant advantage of setting it at 0.1C over 0.05C?

Is there anyone out there who could confirm or give me some advice Please?

Last edited by teleman; 16th January 2013 at 01:22 PM.
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Old 16th January 2013, 03:48 PM   #2
gootee is offline gootee  United States
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In normal operation, your circuit will only draw as much current as it needs. The only reason I can see for the protection circuit is so that when the battery voltage does eventually fall, any constant-power type circuit, such as a switch-mode power supply, would start to increase the current draw, to compensate for the lower voltage. It's sometimes amazing what can get melted, or ignited, under those conditions.

Anyway, about the protection circuit's max current setting: if your circuit never needs to draw peaks of over 155 mA then you should be able to limit it to that. I would tend to want to use the lowest limit that would not adversely affect operation. But check whether or not it needs more during transient events, such as at startup.

Your lithium ion battery voltage should stay relatively constant until the batteries are almost depleted and then the voltage will go down relatively suddenly.
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Old 16th January 2013, 06:59 PM   #3
teleman is offline teleman  Norway
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Thank you indeed for your very precise explanation! There's no smps involved, besides the PSM has full over current,battery drain & short circuit protection,so I am well covered I think.
My only dilemma is whether to choose the discharge current fixed for a 3100mA battery pack or leave a higher margin in case a lower capacity battery such as 2200,2600or a 2800mA cell pack is used. Considering transients taken into account as you've rightly pointed out, won't it be then safer to go instead for 0.1C max discharge current?

So don't you think that 0.1C perhaps would be the safest limit Ishould have? Any thoughts on that?
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Old 16th January 2013, 07:54 PM   #4
dmills is offline dmills  United Kingdom
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0.1C would be what I would set, no point in having over current settings dialled in too close to the line.

Note that the major function of that circuit (Apart from preventing many hundreds of amps of short circuit current from flowing (Yes, really some lithium has that low an internal resistance), is to stop you excessively discharging the pack which can cause the pack to catch fire.

Lithium chemistrys have a fairly narrow range of voltage that they must be maintained within, going outside that range in either direction will usually cause trouble, including smoke.

Regards, Dan.
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Old 16th January 2013, 08:48 PM   #5
teleman is offline teleman  Norway
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Many thanks Dmills,
Exactly my thoughts too! 0.1C seems the most sensible limit. Thanks gootee for pointing me in the right direction.
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Old 17th January 2013, 10:29 AM   #6
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
Are Lithium of the type that each cell must be monitored to ensure each cell never falls below minimum voltage?

6 cells each with a voltage monitor gets quite complicated !

Nicads just used a diode to reduce the reverse voltage applied to the first cell (in a series connected set) that became discharged.
regards Andrew T.
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Old 17th January 2013, 12:19 PM   #7
teleman is offline teleman  Norway
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Not really imho; with a PCM (Protection Circuit module) board, all one needs to do is, pre adjust the max.discharge current on the pcm & solder all the battery terminals to the board. Charging too is done through the pcm. Most of these pcms even have individual "Cell balancing" to control charge & discharge currents among other parameters. PCM also controls minum/max charge & min/max threshold voltages. Very smart

Unprotected cells through a PCM have a distinct advantage over individually protected cells...as you very well know what would happen if one of the cells should die or malfunction in a protected series pack!
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