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Old 02-05-2011, 07:05 AM   #2 (permalink)
redbird300
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1. The general idea:

OK, so some of you might need to discharge lipos from time to time. You charged a bunch of lipos, and rain starts falling down during the whole day for example. Or you can’t go to the field anyway, as the wife suddenly decided that you need to mow the lawn. Bad luck, but we need to deal with these conditions.

Best solution is to avoid the need to discharge at all times, by only fast-charging what you really need at that moment and nothing more, but this scenario is not likely for everyone, or in all circumstances. So let’s see what we could do .

The most common practice must be to power up your lipo charger, put it in discharge mode and push a lot of buttons to pick the discharge rate and end voltage, hook up the pack, and wait until the discharging is finished, which takes a lot of time with most chargers. The discharging capabilities are not very powerful, even some high end chargers allow only for 60-80 Watts discharge capacity. Many smaller chargers only allow for 20 Watts, which takes ages to discharge a large pack: a fully charged 6S/5000mAh pack would need around 3 hours to lose half of it’s charge for example ! I am not talking about regenerative discharge into a car battery here, as this involves even more work to hook it all up. It allows for high currents, but I looked for another solution.

Some people use automotive headlight bulbs to discharge lipos, but even when combined with a timer this involves some risks. There is no safe and 100% sure way to discharge to an exactly predetermined and controlled voltage, and the individual cells are not actively monitored.

Wouldn’t it be nice to have a simple device, not needing to be connected to a power outlet, where you hook up the pack and which discharges fast, automatic, safe and easily ? Well, that’s the general idea. Problem is that I didn’t find these in any shop, so I decided to try to make one myself. Note that the idea is not entirely mine, I simply read about a similar project somewhere, and just made my own version.

What did we want exactly ?

- Something fairly easy to build yourself, given some basic electric knowledge, without complicated self made electronic circuits. This involves a little tric of course, which consists in using a specific lipo monitor with an external alarm port (the Hobbyking “Cell-Log 8”), as you’ll see further down this thread.
- Must be able to reach a decent discharge current, I started with the idea of a 80 Watt prototype, but it will become clear that making a more powerful one is very easy with this design. In fact, just changing the value of one component will be enough.
- Not excessively expensive. Yeah, we need that money for that new heli, lol.
- Independent from any external power supply or electrical outlet. We already have cables enough as it is, and this will allow for field use.
- No active cooling if possible, to avoid noise and extra power problems, a drawback is that we’ll need a very large heatsink for decent passive cooling !
- Programmable settings like end voltages, as well for the whole pack as each individual cell, a safety timer, etc…
- We didn’t pay attention for this first demo version to the casing yet, it’s meant for indoor use only, but anyone wanting to use it outdoors could make a nice case around it.
- Neither did we try to make the device usable for any packs other than 6S lipos, the most popular kind nowadays, right after the 3S packs. As the 3S packs have less capacity, the problem seemed less urgent to me. You could still discharge these with any normal charger in a reasonable amount of time, or even build a second discharger suitable for 3S packs, by using a 12Volt relay, etc…

Impossible ? No, not really, but there are some problems to solve, read on in part 2..
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