Battery charger for wider voltage range
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 16th September 2013, 10:44 AM #1 akis   diyAudio Member   Join Date: Jul 2008 Battery charger for wider voltage range I have bought some Li-ion batteries from China which are not what they purport to be. Outside of the capacity issue (they are less than 1/3 of claimed), they have a very wide voltage range and can discharge to way beyond safe limits. I have constructed a battery charger for a nominal 12V, in practice from about 10V to 13V. However these batteries can drop to as low as 6.5V (I discovered after one year of non use). Currently my battery charger has 4 positions, selecting the charge current, 100mA, 250mA, 500mA and 1000mA. However if the battery has drained down to say 6.5V and you select 1000mA, the charger will need to dissipate more than 10W of heat. **** This question is about ideas to limit the power dissipated as heat. **** The first idea that comes to mind is to try and limit the current when the voltage is too low. Another idea would be to have a variable voltage supply to the charger to follow the battery charge level, so for example if the battery is at 6.5V then the supply to the charger should be 6.5V + internal losses = 11V. If the battery is at 12.5V then the supply should be 12.5V + internal losses = 17V. Maybe there are other ideas or ways of achieving the same, and I do not know how to make a variable DC supply without losing heat anywhere, I mean it seems I will need to have a variable mains transformer ?
 16th September 2013, 01:47 PM #2 sofaspud   diyAudio Member     Join Date: Nov 2005 Location: San Antonio No one else knows how to make a variable DC supply without losing heat anywhere. Perhaps posting a schematic would help potential responders. I for one am a bit confused about what you're requesting. __________________ It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
 16th September 2013, 03:41 PM #3 Osvaldo de Banfield   diyAudio Member     Join Date: Dec 2011 Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina An easy way to avoid power loose is to detect the battery level, and switch the secondary of the transformer with a relay to a lower voltage. As voltage increases, relay gets higher voltage levels. Easy to do using a LM393 double comparator wired as window comparator. Also you need a tap or taps in the transformer's secondary. __________________ Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001. Argentine Ham Radio LW1DSE since 1987.
 16th September 2013, 06:05 PM #4 sofaspud   diyAudio Member     Join Date: Nov 2005 Location: San Antonio It seems like a fan and/or better heatsink would be easiest. What has confused me most is that if the charger puts out a regulated 12V, when connected to a 6.5V battery it will still output 12V. I suppose it is the OP's second to last paragraph that has me stumped; it isn't correct to my mind. Charge voltage must be higher than the battery voltage. If this is a constant-current charger, it needs to be designed for worst case, not kludged with a Variac, etc. __________________ It is error only, and not truth, that shrinks from enquiry. - Thomas Paine Last edited by sofaspud; 16th September 2013 at 06:08 PM.
 16th September 2013, 08:21 PM #5 sghr220 diyAudio Member   Join Date: Aug 2013 Sorry guys but none of the above seems like a practical solution to the original problem... One way to do it is to construct the charger around a low Rds(on) MOSFET like say the IRF540 which has an Rds(on) of 33 milliohm and capable of 33A drain current so power dissipated isn't gonna be a problem if a normal heat sink is used. to control the current passing through the MOSFET you can use a pulse width modulation (PWM) with a fixed frequency and a variable duty cycle dependent on the battery voltage, to generate this signal you can use various techniques micro controllers, 555 timer or even a computer port.
 16th September 2013, 08:30 PM #6 dmills   diyAudio Member   Join Date: Aug 2008 Location: High Wycombe Two words: Buck converter. Linear tech do some sand that is reasonable for this, and you typically get better then 85% efficiency. Seriously a current mode switcher is the way to do this, and is really not hard these days. I would note that charging something claiming to be lithium that has dropped that low should be conducted with extreme caution, fire is very possible. Regards, Dan.
sofaspud
diyAudio Member

Join Date: Nov 2005
Location: San Antonio
Quote:
 Originally Posted by sghr220 Sorry guys but none of the above seems like a practical solution to the original problem...
Wow. Maybe stating the original problem and why none of the above seems like a practical solution would make your response come across a little better.
__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine

sghr220
diyAudio Member

Join Date: Aug 2013
Quote:
 Originally Posted by sofaspud Maybe stating the original problem
That's a question for the OP but i guess if you read his words carefully he makes himself pretty clear.

Quote:
 Originally Posted by sofaspud why none of the above seems like a practical solution would make your response come across a little better.
A "fan or a better heat sink" is hiding the problem with circuit design and not solving it. the problem is heat generated/energy wasted by certain circuit element, solution===>> use another element (and perhaps a better technique) that have a very low internal resistance with current handling capabilities way more than needed. Simple ain't it.

akis
diyAudio Member

Join Date: Jul 2008
Quote:
 Originally Posted by sghr220 Sorry guys but none of the above seems like a practical solution to the original problem... One way to do it is to construct the charger around a low Rds(on) MOSFET like say the IRF540 which has an Rds(on) of 33 milliohm and capable of 33A drain current so power dissipated isn't gonna be a problem if a normal heat sink is used. to control the current passing through the MOSFET you can use a pulse width modulation (PWM) with a fixed frequency and a variable duty cycle dependent on the battery voltage, to generate this signal you can use various techniques micro controllers, 555 timer or even a computer port.
If I understand it correctly this technique lowers the average current when the battery is too drained? Is that not the same as in lowering the charging current (somehow) ?

akis
diyAudio Member

Join Date: Jul 2008
Quote:
 Originally Posted by sofaspud It seems like a fan and/or better heatsink would be easiest. What has confused me most is that if the charger puts out a regulated 12V, when connected to a 6.5V battery it will still output 12V. I suppose it is the OP's second to last paragraph that has me stumped; it isn't correct to my mind. Charge voltage must be higher than the battery voltage. If this is a constant-current charger, it needs to be designed for worst case, not kludged with a Variac, etc.
A battery charger is not simply a regulated X volt supply. It is more like a current limited X volt supply, which means that in many/most cases the current limiting circuitry is active. That further means that when the battery is down to 6.5V, the charger will output something like 6.6V-6.8V on the battery - just what is needed to reach and trigger the current limit. The current limit is chosen by the user, depending on the battery, and the choice of slow-fast charging programme. The voltage limit is also chosen by the user depending on how much strain he wants to put on the lithium-ion battery, the higher the voltage the greater the strain.

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