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Old 16th September 2013, 10:44 AM   #1
akis is offline akis  United Kingdom
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Default 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 ?
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Old 16th September 2013, 01:47 PM   #2
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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.
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Old 16th September 2013, 03:41 PM   #3
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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.
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Old 16th September 2013, 06:05 PM   #4
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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.
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Last edited by sofaspud; 16th September 2013 at 06:08 PM.
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Old 16th September 2013, 08:21 PM   #5
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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.
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Old 16th September 2013, 08:30 PM   #6
dmills is offline dmills  United Kingdom
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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.
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Old 16th September 2013, 08:36 PM   #7
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Quote:
Originally Posted by sghr220 View Post
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.
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Old 16th September 2013, 08:58 PM   #8
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Quote:
Originally Posted by sofaspud View Post
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 View Post
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.
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Old 16th September 2013, 09:39 PM   #9
akis is offline akis  United Kingdom
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Quote:
Originally Posted by sghr220 View Post
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) ?
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Old 16th September 2013, 09:50 PM   #10
akis is offline akis  United Kingdom
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Quote:
Originally Posted by sofaspud View Post
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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