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Old 6th January 2011, 07:06 PM   #1
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Default Li-Ion supply for ghettoblaster

I'm making a smallish ghettoblaster for portable jamming, and I'm looking for a little help designing the power supply.

The general idea is to use a heap'o lithium cells (probably eight cells alltogether) to make a "center tapped" battery bank to give me something like a -12v/0v/12v supply (using a positive and negative buck converter to keep everything at a stable voltage).

I plan on getting the cells with built-in protection circuits, so exploding batteries shouldn't be a big problem.

My big question is about charging. I may either build my own charging circuits with max1551's (or similar), or acquire and rip-up some commercial chargers. My main questions about the chargers are:

1) Can I discharge the batteries while they are charging?

2) The batteries will be connected in series, assuming I use the 1551, there will be one charger per battery, can I connect a charger to each battery while the batteries are still connected in series and charge them like that? Or do I need to find a charger designed for a larger number of batteries in series?
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Old 6th January 2011, 07:39 PM   #2
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1) Depends on the charge circuit - but to error on the side of safety, it's probably not a good idea.

2) If you used one 1551 per cell, each 1551 would have to be powered by its own isolated power supply. If you used only one power supply for all of them then all the batteries would need to be disconnected from each other.

Charging/discharging multiple lithium ion cells in series can be a real hazard if not done properly. There is really no cheap way to do it; you will need to invest in the proper balancing/charging/watchdog circuits.
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Old 6th January 2011, 09:06 PM   #3
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Thanks,for the info

I think given that (sounds like doing it myself with my own cells is expensive/a PITA/dangerous), I might go with some 18v power drill batteries + a buck converter to bring it down to 12v for a t-amp.
These can apparenly be had relatively inexpensively (looks like ~20-30 for a pair of makita 18v 1.5Ah). I believe these batteries have protection circuits built into the pack.

From what I understand, discharging these batteries in paralell is no problem, but how about charging? Can I charge two packs in paralell with one charger, or should I use two chargers?
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Old 7th January 2011, 03:06 PM   #4
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I've been doing some more research, and apparently there are chips out there that appear to do everything required for a li-ion battery, for example the TI bq77PL900. This looks like it would be very straightforward to set up and seems to be a complete solution and allows for 10 cells in series. Seems like this + buck converter could give me a couple hours on a t-amp depending on the exact setup of drivers I settle on. (assuming 4 or 5 Ah cells).

Last edited by hospadar; 7th January 2011 at 03:10 PM.
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Old 8th January 2011, 05:01 AM   #5
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hospadar
are you really size and weight constrained?
If not so just go for NiMH battery. Still, the proper sharging is not that simple, but as you've suggested cordless tool's chargers would be the way to go. You also may use two batteries in parallel by decoupling them with Schottky diodes. Doing so they can be charged/discharged simultaneously, with separate sharging units each. Unfortunately it will require good lytics tied at the point of load. Even without the diodes it would be wise to use lytic, due to the "slowness" of the battery. It would be fare to say that battery itself has substantial Faraday capacitance but behave quite specifically at various frequency load.
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Old 8th January 2011, 06:23 AM   #6
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I do final test on many BQ battery management EVMs.
One and two cell BQs for sure. I've never seen 10 cell!
Not to say something like that won't land on my desk.
I'm usually the last one to know anything.

Anyways, your biggest problem will be shipping Lithium
by air. There are some ugly rules about how many and
how big each cell can be. You might have to make many
small buys of three LiPo (safest type) cells or less. Else
its coming to you only by slowest boat, train, or truck.

I don't see any need to buck down a stack of cells to
12V. Good switching amp (again suggesting TI of course,
yeah yeah I test those too) can take down the voltage.
Would achieve same result from perspective of the load.
-12 also not needed for a bridge tied load.

I'm no engineer, just a test tech. Also no employee of TI,
only a contract manufacturer down the street. Still, if I
can help...

Last edited by kenpeter; 8th January 2011 at 06:47 AM.
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Old 8th January 2011, 08:36 AM   #7
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thanks for the tips, updates:
I got some 4 Ah cells on eBay and found a 10 cell charger for $30 ish. Combined with free samles from TI it's going to be pretty affordable compared to a pre-fab pack.

As to the buck converter I have a t-amp that takes plain 12vdc, but maybe I'm missing something here (it's the standalone t-amp that parts express sells). Either way I got some buck converter sample ics from national semi, so if it turns out I don't need them, no big deal.

Also, I plan on setting everything up so that when the whole thing is plugged in the batteries are disconnected from the amp (with some mosfets probably) and the charger is turned on, then the amp will get powered by the mains (via ordinary power supply)

Thanks again for all the help
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