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Lead-Acid Battery Chargers - Choosing the Correct Battery Management/Charging System

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AJBotha

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Hi Guys

I am working on a project that is in essence a "smartish" power supply bus. The AC/DC is managed by an off the shelf PSU, so no real work there.
A microcontoller controls power to 4 sets of downlinks, controlled by FETs. If something goes faulty down the line I can switch off the supply to that bus and notify the system admins.

The supply voltage (read: bus output) should be between 48V and 55V. 48V seems cheaper at this stage and I suspect I will go for a 48V bus (DC).
The power supply has back-up lead-acid batteries, they need to be charged. If I went with the 55V bus I could series 4x 12V (48V battery backup) batteries and still have enough left (from AC/DC supply) to charge them with.
If I go for the 48V bus, I wont have enough voltage to charge the array effectively. The I will look at 24V battery backup, which is also acceptable for the output bus, even though less efficient.

My actual question is how do I charge the backup batteries - options seem either very inefficient or horribly complicated! I think a switching battery charger IC is the way that I would like to go.

My requirements would be mostly maintenance charging (float charging) and if the batteries go below Vmin to charge them at 1A or whatever. I am more interested in a smart/efficient maintenance charger than a 5 minute 50Amp charger! Do you have any leads that I can investigate?

The Texas Instruments BQ2031 seems easy enough to use, but it can only work from 5V, a voltage I don't have available and don't necessarily want to introduce for a single IC.

The Linear Technology options seem pretty complicated and probably expensive, and also seem to require more external parts. But it can operate from supply voltage up to 55V.

If you can maybe help, please do. I am inexperienced but eager to learn!:popcorn:
 

SunnySkyguy

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re: Lead-Acid Battery Chargers - Choosing the Correct Battery Management/Charging System

If you have a remote uC to control the switches, then you can also supply 5V power and perhaps enough to boost to 52V for float charge @10W with a pulse on top.

Pulse power is the preferred way to prevent sulfation on idle backup batteries.

So the TI part seems like a good choice if you use a local AC/DC charger.

Otherwise you need to define the power required to recharge power lost during down time , assume 50% storage efficiency and then recovery time need for next outage.
 

AJBotha

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re: Lead-Acid Battery Chargers - Choosing the Correct Battery Management/Charging System

If you have a remote uC to control the switches, then you can also supply 5V power and perhaps enough to boost to 52V for float charge @10W with a pulse on top.

Pulse power is the preferred way to prevent sulfation on idle backup batteries.

Can you please elaborate or explain this part of your answer?

I do have a 3V3 net locally for the micro, but that TI IC can only operate from 5V.
That IC will use very little power, right?
Maybe I can drop some voltage over a series resistor and then use a 5V zener?

Then we are starting to go down the inefficient line again, which I would like to steer away from!
 

BradtheRad

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re: Lead-Acid Battery Chargers - Choosing the Correct Battery Management/Charging System

My requirements would be mostly maintenance charging (float charging) and if the batteries go below Vmin to charge them at 1A or whatever. I am more interested in a smart/efficient maintenance charger than a 5 minute 50Amp charger! Do you have any leads that I can investigate?

Maintenance is a different circuit than a 50A charger. The variation of voltage might be less than a volt (on a 12V battery).

Possible options:

(a) Wait until bat. V drops to maybe 12.3V, then apply a few A to bring it up to 12.8, then stop charging. This voltage range can be detected by a couple of op amps, and a zener or two.

(b) Charge with a few A once a week, to ensure batteries are always close to full charge. It should be possible to program your microcontroller to keep track of time.

(c) Apply a 'trickle' charge continuously, to counteract the battery's tendency to self-discharge. A common amount is 1/4 A. However it ought to be fine-tuned over a long term, since you don't want to overcharge, nor turn electrolyte to gas.
 

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