Battery discharger - RC-Monster Forums

BrianG · **Battery discharger -** 04.14.2006, 10:22 PM

Hey, I was looking at some battery dischargers and found them to be a bit expensive for the "smart" ones.

I don't like the light-bulb types since there is no shutoff. Plus, the discharge rate changes as the battery is discharged. As the battery gets weaker, it can no longer light the filaments. Unfortunately, the resistance of a lightbulb filament is quite a bit lower when it is not lit. So, at the weakest state of a battery pack, it pulls the most current (or tries to) because of the low bulb resistance.

So, I was designing a solid-state design (using transistors and heatsink) that would discharge at a constant current no matter the charge level and then shut off at a pre-determined cell voltage level. I don't know an easy way to make it smart enough to know how many cells the pack has so there is a switch to select the number of cells (5, 6, 7, or 8 cells). There is another switch that selects the charge current (5, 10, 20, and 30A). The cutoff voltage would be set for 0.8v/cell (this value can be easily fine-tuned). There is even a discharge LED and a "done" LED. It works good "on paper", but was wondering if there truly is a benefit to discharging packs before charging them again? This project will only cost about $50 to build - the most expensive part being the heatsink, but I want to make sure it's worth it.

HotnCold · 04.15.2006, 05:03 AM

There is a discharger shutoff for bulb dischargers. I have one and it works very well - I will try to get you the info for it. The only thing that it dosent do is bring down each cell individually. They have individual cell dischargers - but they can get expensive also.

squeeforever · 04.15.2006, 08:41 AM

the new teken one is nice. think rchippie has some.

Sylvester · 04.15.2006, 08:51 AM

Why not just run the batteries in a truck and charge after ? :005:

BrianG · 04.15.2006, 02:06 PM

Quote:

Originally Posted by Sylvester

Why not just run the batteries in a truck and charge after ? :005:

Funny guy. :) Of course I only run the batts down to when the truck slows down. But I've read that if you fully discharge to the max safe value, and then recharge them, you get a better charge and performance.

Making a voltage cutoff add-on to the light tree would be extremely simple. However, it doesn't solve the uneven discharge rate the light tree offers at various states of battery discharge. Besides, making a solid state one would have adjustable cell counts, discharge currents, be more durable, and just simply cool. :)

I can post the schematic if there are any electronics-minded people here. Maybe I could get some feedback on the design?

coolhandcountry · 04.15.2006, 04:07 PM

To my understanding I thought that was nicad. I thought the nimhs was not nessecary. I could be wrong. Probably to. ;)

maxxdude1234 · 04.15.2006, 04:16 PM

You don't have to discharge NiMH's fully before you charge them, but you'll get a 'better' charge if you do. It's the NiCAD's that get memory if you don't discharge them, and they begin to loose their capacity and power.

BrianG · 04.15.2006, 05:18 PM

I originally thought so too, but this link says otherwise: http://en.wikipedia.org/wiki/NiMH

The part that I found especially interesting is:

Quote:

Quote excerpt from Wikipedia.com
A single cell driving a load generally cannot become reverse charged, simply discharging down to 0 volts. This does not cause irreparable damage, and in fact if performed periodically and followed by a full recharge will help to maintain the cell's capacity and performance.

From this, it appears the danger of cell reversal is only present if a number of batteries are connected in series. So, to be safe, discharge to about 0.8v when not discharging a single cell.

There are several very informative links at the bottom of the article. I love this site. :003: