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Old Jan 11, 2006, 10:27 PM
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want some ideas for cheap chargers/method on many cells at once.

I am relatively familiar with lipo and stuff, read a lot on here, searched, can't find exact info...

Ok lets say I have 12 or more vehicles that all run on 3.7volts...
I want to convert them all from nimh to lipo to increase run time...
I may keep it 1S1P lipo or use 1S3P lipo...
I may just give up and keep the nimh LOL...

What I want to do with these vehicles is keep half of them running, and half of them on the charger, so I want to find some lower cost way of charging 6 to 12 of them at the same time... so they can all be ready and done charging at the same time, paying 100$ per charger is not an option when buying 12 chargers lol...

I would like the chargers to be simple as cell phone just plug the vehicle in and be done with it... reason for this I am actualy pondering hacking surplus cell phone chargers... but not sure wich to use.

Draining unbalanced cells in series can cause problems...
I asume the same problem if they are in parralel? less or more of a thing to worry about? less likely or more likely to happen to the cells?

I have read lots about charging in parallel, but im still confused lol... on benifits disadvantages to doing this... im gona use them in par, so I might as well charge them in par... what should I be alert to?

I also need a low cost sorce on lipo cells that can replace a 4 cell 1/2AAA pack (2x2). Pondering the 140-170mah cells from plantraco in a 3P config... but would rather have just one cell to avoid balancing issues in each vehicle... they make thicker lipos?
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Old Jan 12, 2006, 02:20 AM
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hello,

i whould strongly recommend you keeping your NI-MHs for safety and cost reasons, if you don't need light weight in an aeroplane/helicopter.

You can charge series NI-MHs using a 1N4007 diode in series with an LM317, output series with resistor, returned to ADJ pin, this is a fairly stable Current Generator, set it to C/10.

I=1.25V/R
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Old Jan 12, 2006, 06:06 AM
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You can charge 100 LiPoly cells in parallel at the same time if you want to. Cells charged in parallel do not become out of balance .

Everyone want low cost. I bet if you call a dealer and tell him you want to purchase $10,000 worth of LiPolys and pay in cash he will give you a good discount.

Charles
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Old Jan 12, 2006, 06:42 AM
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For large numbers of single LiPo cells, single-cell chargers are your safest option. There are plenty of them around. The type you mention is one good possibility.

Second best, IMO, charge the cells in parallel. What size cell are you using, e.g., 2000 mAh?

The thing you will need to look out for is overamperage charging this or that cell in the parallel array. Use a clamp-on ammeter to measure amperage to each cell, that's all.

- RD
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Old Jan 12, 2006, 07:07 AM
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Maybe just connecting the cells thru resistor ( each cell its own resistor, of certain value to keep current below 1C with up to 0.6V voltage diference 4.2V - 3.6V), and quality hard and precision lab power supply set to 4.2V (4.15 better). It will solve this problem, and You can connect any number of any state of charge cells. Full Charge should take about 2 to 3 hours, but maybe it is acceptable.
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Old Jan 12, 2006, 02:09 PM
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I guess what I want is just something to plug the lipo pack into (each vehicle needs to be seperate), and have it just cut off at the 4.15 volts... is this what cell phone chargers do?

I am replacing a 3 cell 120mah nihm pack... so basicly I am trying to use the smallest lipo I can find... but I NEED more than 120mah run time... using 3 170mah in par, will give me 510mah run time, and that is about what I need.

I am also now pondering what I will do to make sure the vehicle cuts off before the lipo is over drained.. I already think the vehicle stops working well above the safe voltage, but I will have to make sure... a mini lipo-saver i guess. They should just build a 2 way lipo saver into every pack... stop it from over chargeing stop it from over discharging... kinda like those rayovac IC3 cells .
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Old Jan 12, 2006, 02:32 PM
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Quote:
Originally Posted by coro
Maybe just connecting the cells thru resistor ( each cell its own resistor, of certain value to keep current below 1C with up to 0.6V voltage diference 4.2V - 3.6V), and quality hard and precision lab power supply set to 4.2V (4.15 better). It will solve this problem, and You can connect any number of any state of charge cells. Full Charge should take about 2 to 3 hours, but maybe it is acceptable.

if you want to charge series LI pack, you should be worried about balancing, not over voltage, anyway, 4.15 or 4.2 ? not much difference, there is large voltage drop on the wires to the LI cell too ..., unless you connect the sense probes directly to the LI cell terminals.
Of course this voltage drop decreases when charge current decreases, but it is still there, at fully charged cell, termination of 0.1C, 3500mAh pack, you have 250mA flowing at 4.20V, so if you have a wire resistance of 28mOhms, then this will equal to a 9.8 mV voltage drop ! and this is a fairly low wire resistance, you got contact resistances at the connectors...

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Old Jan 12, 2006, 03:13 PM
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Quote:
Originally Posted by z-matrix

if you want to charge series LI pack,....
No, i mean single cells (1s packs) and charge will be PARALEL, common ground and resistors leading to voltage source 4.20V.
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Old Jan 13, 2006, 04:58 AM
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Originally Posted by coro
No, i mean single cells (1s packs) and charge will be PARALEL, common ground and resistors leading to voltage source 4.20V.
In this case you could set the current limit on your supply to 1C, and you don't need resistors
but be sure you have 4.20V +-50mV tolerance at least at the voltage generator part.

But you have to measure the current, as it willl not automatically stop charging at 0.1C amps.
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Old Jan 19, 2006, 01:47 AM
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OK.. going to temporarily stick to nimh... are there any low cost 3cell NIMH chargers out there? the charger these thigns come with is just a timed charge, and thats just not acceptable to me... I want it to turn off when its done, but the cells are so small that my triton charger and other chargers tend to false peak.. even when set to a high tollerance setting...

I need a 3 or 4 cell peak charger at about 100-700mah range.

With any luck I wont need the home made lipo chargers...
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Old Jan 19, 2006, 05:17 AM
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Quote:
Originally Posted by z-matrix
In this case you could set the current limit on your supply to 1C, and you don't need resistors
but be sure you have 4.20V +-50mV tolerance at least at the voltage generator part.

But you have to measure the current, as it willl not automatically stop charging at 0.1C amps.
at first, You cannot connect one empty cell just in paralel to 10 another paraleled allmost full cells.
At second, lab supply should have precision voltage and may have infinite current to supply any number of cells.
Automatic stop is not so necesary, I know that charge should stop at lower than certain current, but many users of linear chargers without timeout or stop feature did never feedback any troubles with it.
Nice DIY sollution could be created by limiting current at near zero voltage drop, using some fet transistor and oz, but it is questionable if it is worth the spent time.

Draconious, fast (less than 4 hour) Lipo charge is much easier than Nicd / NiMH. Of course, voltage is critical with Lipos. But NiMh charger have to be at least 12 hour (1/10C) to be simple - otherwise deltapeak detection is required to avoid overheating them..
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Last edited by coro; Jan 19, 2006 at 05:29 AM.
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