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Battery discharge load.

Battery performance and checking via discharger

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David Hall 922/02/2020 19:35:29
201 forum posts
14 photos

I rigged up this car headlight bulb to discharge my 3s & 2s batteries. I need to watch them to ensure they don't go too far, or use my cheap battery level alarm gadget (even so, I have ruined a couple that I left too long!).


The battery load has been very useful, so I began to look at the ready made dischargers that could take over 150W. They become more expensive, so I looked for a way to make my own.

Here is the result of several rainy weekends playing with bits..(still a work in progress). A quad gang of four x 60W bulbs (should be 100W bulbs but turned out to be less), held in tin cans (the bulbs get very hot and need a metal mount), cooled by a fan and shrouded so as to not be within my vision (they can be bright!). The load is regulated by a brushed ESC (it's like a dimmer) worked by a servo tester to give the throttle input. quadgang.jpg

The 3D printed frame works well in holding it all together.

David Hall 922/02/2020 19:43:00
201 forum posts
14 photos

With this setup, I can test/torture some of my smaller batteries at or near their C rating. My F5J gliders push smaller batteries hard, a failure in the climb out is a possibility, so this can help find poor cells early. It has shown the performance differences between different batteries. My 450mAh/3s/65C get a hard time at 140W discharge even though they should be good for much more. The 500mAh/3s/65C Bolts work very well in this test.

It's better with the hood... hoodedbulbs.jpg


Edited By David Hall 9 on 22/02/2020 19:47:13

Gary Manuel22/02/2020 20:23:01
2186 forum posts
1540 photos

A much better way of discharging your batteries is to put them into your models and fly them.

I'm glad it's not just me suffering with cabin fever due to this dreadful weather. Hopefully it will improve soon, so that we don't all need to think of wacky ways to fill our time.

PS do I need to use any particular type of can? Will Heinz beans cans work?

David Hall 922/02/2020 20:42:36
201 forum posts
14 photos

Quite right, flying them to storage levels would be best, but then I might miss the chance to torture a few....

I would like to tell you that beer cans work best, but alas.... several small cans of beans were sacrificed to make the gizmo. The single bulb is on the big can for sweetcorn, also a contributor to... er... "restlessness"

PatMc22/02/2020 21:09:12
4318 forum posts
524 photos

Why not use a brushed motor, say 600 can type, fitted with a prop that would take about 20A @WOT and use an ESC to regulate it to 150+W ?

Edited By PatMc on 22/02/2020 21:10:20

Devcon122/02/2020 21:27:52
1400 forum posts
489 photos

Does a charger/discharger perform a similar function.

Dickw22/02/2020 23:06:44
601 forum posts
89 photos

Some (most?) chargers/dischargers can be used to discharge at higher than normal currents by putting a resistor (e.g.. car headlight bulb) in series with the battery main connections during discharge.

The charger monitors the battery via the balance lead so will switch off the discharge at the appropriate time, but much of the discharge load is dissipated in the resistor rather than the charger so you can safely go well over the charger's specified discharge rating.

The process is described in the i-Charger manual, but I have found it works on other chargers as well.

For even higher discharge currents I just use the normal motor/ESC/prop with data logging/telemetry to monitor the process.


MattyB22/02/2020 23:26:54
2011 forum posts
31 photos

There are some nice (and pretty cheap) commercial units that do this - I have rigged up 3 in parallel in line with this thread to give a 150W discharge capability. Works great!

Peter Jenkins23/02/2020 00:15:33
1371 forum posts
138 photos
Posted by David Hall 9 on 22/02/2020 20:42:36:

Quite right, flying them to storage levels would be best, but then I might miss the chance to torture a few....

I don't understand what you want to achieve by torturing a few packs! As already suggested, flying them will give you all the information you need especially if you use on-board telemetry to give the essentials of A, V and W plus charge remaining. I fly 10S 5000 packs where max current draw is around 2,800 to 3,000 W. If you have a duff cell your battery charger will show that or else when you check the cell voltage after flight.

If you have blown a few packs with your testing regime that seems an expensive way of going about proving that you have/had a good/bad pack.

David Hall 923/02/2020 09:41:10
201 forum posts
14 photos

I plan to use the unit for simple discharging to storage voltage, often groups of batteries. Running a motor to do this would be noisy and require very close attention.

It's main intended use is to run batteries at or at least near the rate at which they run. For most batteries/aircraft that is not a fixed figure, but for the tiny batteries in my F5J gliders, they run at a high rate for a short time, then a low rate, just powering the RX system. I have no idea how to check over packs that are capable of much higher power.

I've had new batteries that are poor performers, even though they appear to charge perfectly well. I believe that relying on the charger to indicate a battery fault is not good enough. The fact that a battery achieves a balanced charge is not an indication that it is a good battery, or even that it is at full capacity.

Peter Jenkins23/02/2020 10:37:20
1371 forum posts
138 photos

Most of today's LiPo battery chargers have a charge/discharge to Storage voltage without resorting to running a pack down with either a motor or a load. This is handled by the Charger software so you won't accidentally discharge the LiPo pack below 3.0 v/cell.

Each discharge at high current will decrease the battery life as if it has been used in the air so, given the expense of batteries, not a great idea IMHO.

Poor performing batteries are a fact of life and make themselves known as you use them.

I certainly wouldn't recommend others using your system given the danger of dropping cells below 3.0 v. An on-board telemetry package will give far better information on the health of your pack when tied in with the information from your charger on how much was put back into the pack.

Each to their own though.

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