Getting my head round discharge rates - help please

[Witterings]

I've had a long break from flying and am trying to get my head back round discharge rates again for storage charging unused batteries.

 

I know if I have a battery that's say 2000 mah or 2 amps and I set the charger to charge at 2 amps that's a 1c charge rate but if I was going to use a ISDT FD 200 who's minimum discharge rate is 5A with a small 500 mah battery or 0.5 amp, as it doesn't ask capacity (I don't think) it's going to be discharging it the equivalent of 10 amps or 10 C ... have I worked that out correctly?

 

Whilst I appreciated flying will probably put the same load on it and the ESC is 12 amps but it seems quite harsh to discharge a Lipo for storage at that rate if it's not necessary.

 

Also if I've got one of the discharging setups that uses 3 x 50 watt bulbs = 150 watt .... to work out the amps do I divide the 150W by the battery voltage i.e. 11.1v if it's 3s (I appreciate the voltage will vary from an initial 12.6 to 11.1 as it discharges) which is approx. 13.5 amps so again using a 2000 mah battery it'd be a C rate of 6.75 but if it's the 500 mah battery, it'd be the equivalent of a 27 amp or 27 C draw.

 

Have I got any of that right or completely gone off on a tangent ... as I'm sure you can appreciate help may be needed 😄

[Simon Chaddock]

Surely the charger allows you to set the discharge rate just as it does for the charge rate.

You wont go far wrong with a 1c discharge rate.

The really important bit is the voltage cut off level. Too low a voltage will likely damage a LiPo, particularly if the storage is long term.

The object of a storage charge is to reduce the chemical stress on the cell and to limit the total energy stored to reduce the likely hood of a thermal runaway. 

It is not uncommon for a charger to stop at 3.85V per cell. It does this with the balance lead connected so it actually "balance" discharges each cell to exactly the same voltage and leaving enough capacity to cover a couple of years self discharge before the critical 3.7V/cell is reached.

I hope this helps.

  

[Witterings]

1 hour ago, Simon Chaddock said:

Surely the charger allows you to set the discharge rate just as it does for the charge rate.

You wont go far wrong with a 1c discharge rate.

 

The ISDT discharger I mentioned above, you can set the number of cells and the discharge rate but the lowest setting is 5 amps which is fine for a 5000 mah battery and will be a 1 C discharge rate I use 500 mah uite a lot and like you would prefer to be able to discharge at 1 C or even lower as I'm not really in a hurry and would rather put as little stress on the battery as possible.

[Philip Lewis 3]

For small batteries discharge using your charger rather than a dedicated discharger, it will be perfectly adequate for the smaller packs and let you set a low rate of discharge.

 

Setting it to storage charge will bring batteries up or take them down depending on what is needed.

Edited January 13 by Philip Lewis 3
Addition

[Dale Bradly]

If the minimum discharge rate using that appliance is 5A, and you don't wish to discharge your 500mAh batts at 5A, then you will need to come up with a different method of discharging.

 

3 hours ago, Witterings said:

I know if I have a battery that's say 2000 mah or 2 amps and I set the charger to charge at 2 amps that's a 1c charge rate but if I was going to use a ISDT FD 200 who's minimum discharge rate is 5A with a small 500 mah battery or 0.5 amp, as it doesn't ask capacity (I don't think) it's going to be discharging it the equivalent of 10 amps or 10 C ... have I worked that out correctly?

 

Have you worked that out correctly: No.

 

"discharge rate is 5A"  does not equal "discharging it the equivalent of 10 amps"

 

"C" and "amps" although related, are not the same thing.

 

Here's your sentence edited:

... if I was going to use a ISDT FD 200 who's minimum discharge rate is 5A with a small 500 mah battery or 0.5 amp, as it doesn't ask capacity (I don't think) it's going to be discharging it at 5A, in this case 10 C ...

 

FWIW, i wouldn't have a problem with discharging it at 5A. As you indicate, you probably put a higher load on it when it's working it's regular day job. 

 

 

3 hours ago, Witterings said:

Also if I've got one of the discharging setups that uses 3 x 50 watt bulbs = 150 watt .... to work out the amps do I divide the 150W by the battery voltage 

 

Yes

 

 

3 hours ago, Witterings said:

 11.1v if it's 3s ... which is approx. 13.5 amps so again using a 2000 mah battery it'd be a C rate of 6.75 but if it's the 500 mah battery, it'd be the equivalent of a 27 amp or 27 C draw.

 

Again, the same C does not equal Amps: 

11.1v if it's 3s ... which is approx. 13.5 amps so again using a 2000 mah battery it'd be a C rate of 6.75 but if it's the 500 mah battery, it'd be a 13.5 amp or 27 C draw.

 

 

[Witterings]

18 hours ago, Philip Lewis 3 said:

For small batteries discharge using your charger rather than a dedicated discharger, it will be perfectly adequate for the smaller packs and let you set a low rate of discharge.

 

Setting it to storage charge will bring batteries up or take them down depending on what is needed.

 

Thanks for that and do appreciate you can do it with a charger, I bought a relatively expensive one quite a few years back but one thing electronics doesn't tend to like is heat which discharging generates so I#d rather use a cheaper specifically designed alternative that i iot ever goes "pop" I'm not concerned.

 

18 hours ago, Dale Bradly said:

Have you worked that out correctly: No.

 

"discharge rate is 5A"  does not equal "discharging it the equivalent of 10 amps"

 

"C" and "amps" although related, are not the same thing.

 

Here's your sentence edited:

... if I was going to use a ISDT FD 200 who's minimum discharge rate is 5A with a small 500 mah battery or 0.5 amp, as it doesn't ask capacity (I don't think) it's going to be discharging it at 5A, in this case 10 C ...

 

FWIW, i wouldn't have a problem with discharging it at 5A. As you indicate, you probably put a higher load on it when it's working it's regular day job.

 

Thank you for that, I also had a brain fart whilst typing itand didn't quite put what I meant but your reply helped me step back from it and work out what I was trying to say and help get my head round it.

18 hours ago, Dale Bradly said:

 

22 hours ago, Witterings said:

Also if I've got one of the discharging setups that uses 3 x 50 watt bulbs = 150 watt .... to work out the amps do I divide the 150W by the battery voltage 

 

Yes

 

I decided to test this today and used a couple of 3s 1500's I had to hand, I thought it'd discharge at 150w divided by approx 12V = 12.5 amps, I saw in another forum where someone said a single one of these setups will only discharge at 4 amps.

 

I attached a watt meter and it started at 4.2 amps and 50 W and reduced as the voltage fell, ay idea why that is?

 

As you say maybe I shouldn't be concerned discharging at 5 amps even for smaller ones and often it'll be 2+ batteries on a charge board which will make a big difference but as I was testing the light bulb setup I tried it with 2 bulbs and 1 and saw the current draw drop exactly as you'd expect and came down to 1.2 amps.

 

Whilst that will satisfy my needs / put my mind at rest for discharging I#m interested in understanding the math / formula behind it, any idea why it's 4.2 amps and 50 watts max as opposed to 12.5 amps and 150 watts, I'll post a link below to the setup I'm talking about.

 

https://www.aliexpress.com/item/1005006911809411.html?src=google&pdp_npi=4%40dis!GBP!28.60!9.49!!!!!%40!12000038703081158!ppc!!!&src=google&albch=shopping&acnt=494-037-6276&isdl=y&slnk=&plac=&mtctp=&albbt=Google_7_shopping&aff_platform=google&aff_short_key=UneMJZVf&gclsrc=aw.ds&&albagn=888888&&ds_e_adid=&ds_e_matchtype=&ds_e_device=c&ds_e_network=x&ds_e_product_group_id=&ds_e_product_id=en1005006911809411&ds_e_product_merchant_id=766351764&ds_e_product_country=GB&ds_e_product_language=en&ds_e_product_channel=online&ds_e_product_store_id=&ds_url_v=2&albcp=17859500389&albag=&isSmbAutoCall=false&needSmbHouyi=false&gad_source=1&gclid=Cj0KCQiA4fi7BhC5ARIsAEV1YibomkLE9wmhXefV5FfqrLjJYNA3H-s4qOTemV6Ip6xIFmXoO_jTbhoaAic3EALw_wcB

 

[RottenRow]

33 minutes ago, Witterings said:

I decided to test this today and used a couple of 3s 1500's I had to hand, I thought it'd discharge at 150w divided by approx 12V = 12.5 amps, I saw in another forum where someone said a single one of these setups will only discharge at 4 amps.

 

I attached a watt meter and it started at 4.2 amps and 50 W and reduced as the voltage fell, ay idea why that is?

 

What was the voltage of the LiPo at the start of this test? Your wattmeter reading suggests that it was around 12V (50/4.2).

 

I'm assuming that the three 50W lamps are connected in parallel, and that all three lit up brightly on your test. Are you sure that they are all 12V 50W lamps?

 

If they are car lamps, they might actually be rated at a little above 12V though that wouldn't account for such as variation as you are reporting.

 

Brian.

[Witterings]

5 hours ago, RottenRow said:

Your wattmeter reading suggests that it was around 12V (50/4.2). 

 

Bang on the money ... I charged them to test "the rig" and thought they don't need to be fully charged but just a reasonable amount above the 3.85 per cell cut off and the total was 12v.

 

5 hours ago, RottenRow said:

I'm assuming that the three 50W lamps are connected in parallel, and that all three lit up brightly on your test. Are you sure that they are all 12V 50W lamps?

 

I believe they're each 24V and 50W and yes connected in parallel.

Edited January 14 by Witterings

[RottenRow]

11 minutes ago, Witterings said:

I believe they're each 24V and 50W,

Well there is your issue. You are running 24V lamps on 12V… they will consume much less than 50W each on half of their design voltage.

 

If they were linear loads (which they are not) they would only consume 12.5W each lamp. You can’t work out the actual consumption apart from measuring voltage, current and power, as the lamps are operating outside of their specifications.

 

They will of course still work as an effective discharge load. But the above explains why you didn’t measure the high rate of discharge that you initially expected. I would have imagined that the lamps only glowed dimly.

 

Brian.

[Witterings]

21 minutes ago, RottenRow said:

Well there is your issue. You are running 24V lamps on 12V… they will consume much less than 50W each on half of their design voltage.

 

Again, so I can understand it better, if I was discharging 6s batteries at 24v ... does that mean the bulbs would work to their 50W each and it'd still discharge them at 4.2 amps as the wattage would increase?

 

23 minutes ago, RottenRow said:

I would have imagined that the lamps only glowed dimly.

 

They vary during the process between one coming on and the they all do, to say they're dim .... possibly not but I've obviously not seen what they'd look like if they were pushed to their limit which may be very different. 

[AndypC]

You’re maybe overthinking it - the charger will almost always determine the discharge rate. When you think about it, where does the energy dissipate to??? - it’s certainly not a motor! If you want to discharge lipos quickly, you’d need a purpose made discharger

[AndypC]

On 13/01/2025 at 19:28, Simon Chaddock said:

Surely the charger allows you to set the discharge rate just as it does for the charge rate.

You wont go far wrong with a 1c discharge rate.

The really important bit is the voltage cut off level. Too low a voltage will likely damage a LiPo, particularly if the storage is long term.

The object of a storage charge is to reduce the chemical stress on the cell and to limit the total energy stored to reduce the likely hood of a thermal runaway. 

It is not uncommon for a charger to stop at 3.85V per cell. It does this with the balance lead connected so it actually "balance" discharges each cell to exactly the same voltage and leaving enough capacity to cover a couple of years self discharge before the critical 3.7V/cell is reached.

I hope this helps.

  

No - the charger does not let you set the discharge rate as it has the responsibility (as it were) to dissipate the energy…not you. So by all means set the discharge rate to whatever you want but the charger will determine the rate used based on its construction, heat dissipation, heat sinks, cooling etc

[RottenRow]

9 hours ago, Witterings said:

Again, so I can understand it better, if I was discharging 6s batteries at 24v ... does that mean the bulbs would work to their 50W each and it'd still discharge them at 4.2 amps as the wattage would increase?

One 50W lamp run at 24V would draw 2.1A (50/24), so if all three lamps lit the current draw would be 6.3A.

 

I don’t know what the black box does, whether it just measures current to the lamps or if it steps up the battery voltage to 24V to increase the discharge rate. There isn’t much information given.

 

Brian.

[Phil Green]

17 hours ago, Witterings said:

I'll post a link below to the setup I'm talking about.

 

https://www.aliexpress.com/item/1005006911809411.html?src=google&pdp_npi=4%40dis!GBP!28.60!9.49!!!!!%40!12000038703081158!ppc!!!&src=google&albch=shopping&acnt=494-037-6276&isdl=y&slnk=&plac=&mtctp=&albbt=Google_7_shopping&aff_platform=google&aff_short_key=UneMJZVf&gclsrc=aw.ds&&albagn=888888&&ds_e_adid=&ds_e_matchtype=&ds_e_device=c&ds_e_network=x&ds_e_product_group_id=&ds_e_product_id=en1005006911809411&ds_e_product_merchant_id=766351764&ds_e_product_country=GB&ds_e_product_language=en&ds_e_product_channel=online&ds_e_product_store_id=&ds_url_v=2&albcp=17859500389&albag=&isSmbAutoCall=false&needSmbHouyi=false&gad_source=1&gclid=Cj0KCQiA4fi7BhC5ARIsAEV1YibomkLE9wmhXefV5FfqrLjJYNA3H-s4qOTemV6Ip6xIFmXoO_jTbhoaAic3EALw_wcB

 

https://forums.modelflying.co.uk/index.php?/topic/53003-storm-rx/&do=findComment&comment=960544

 

[Nigel R]

Running an incandescent bulb at half its rated voltage results in something like 40% of full power, IIRC.

 

You would need to measure the bulb with a multimeter to get an accurate figure though.

[Simon Chaddock]

Witterings

To answer your question why the amps through a bulb falls away as the voltage of the battery reduces I am afraid you have got to go back to a basic physics formula.

Watts = Volts x Amps

So a 50W bulb at 24V will pass 2.1A

But this only applies at 24V. The bulb is 50W only at that voltage

In basic terms the Wattage of a bulb is proportional to the applied voltage.

Halve the voltage and it will only give half the wattage (approximately) 

So at 12V the same bulb will only pass 1A and will glow pretty dimly.

 

As suggested measurement with a suitable multimeter is the best way to know exactly what the amps are.

 

I still find is surprising that you cannot set the discharge rate with your charger. How does it know what capacity battery you are connecting to it? 

A 5000mAh LiPo can safely handle 10 times the discharge rate of a 1500mAh one. Apply too high a discharge rate to a LiPo it will get hot, be damaged and could burst into flames.

My humble charger is limited internally to a maximum discharge rate of just a 0.5A but you can still adjust that rate to as low as 0.1 A if you are discharging a really tiny LiPo.

[Witterings]

4 hours ago, Simon Chaddock said:

I still find is surprising that you cannot set the discharge rate with your charger. How does it know what capacity battery you are connecting to it? 

 

I mentioned in the opening post I was querying about an ISDT FD 200 which is a dedicate discharger as opposed to a charger and the query was mainly relating to that, it allows you to choose if it's 2s / 3s etc. and you to set the discharge amps but the lowest setting is 5 amps. 

 

[Jason Channing]

I have and use 2 of the ISDT FD 200 dischargers and a 5 amp discharge on a 500 mah pack is fine, (Which is a 10C which is probably a lot less than the motor would be using,) 

[AndypC]

On 16/01/2025 at 10:06, Simon Chaddock said:

Witterings

To answer your question why the amps through a bulb falls away as the voltage of the battery reduces I am afraid you have got to go back to a basic physics formula.

Watts = Volts x Amps

So a 50W bulb at 24V will pass 2.1A

But this only applies at 24V. The bulb is 50W only at that voltage

In basic terms the Wattage of a bulb is proportional to the applied voltage.

Halve the voltage and it will only give half the wattage (approximately) 

So at 12V the same bulb will only pass 1A and will glow pretty dimly.

 

As suggested measurement with a suitable multimeter is the best way to know exactly what the amps are.

 

I still find is surprising that you cannot set the discharge rate with your charger. How does it know what capacity battery you are connecting to it? 

A 5000mAh LiPo can safely handle 10 times the discharge rate of a 1500mAh one. Apply too high a discharge rate to a LiPo it will get hot, be damaged and could burst into flames.

My humble charger is limited internally to a maximum discharge rate of just a 0.5A but you can still adjust that rate to as low as 0.1 A if you are discharging a really tiny LiPo.

Hi Simon - think about it for a second…the charger you’re referring to is a physical device with constraints on space within the housing, fan size and component temperature thresholds. Unless you use a dedicated discharger which will have heavy multiple heat sinks within its housing and a bigger, faster exhaust fan and different components, the charger will determine how quickly it can discharge a battery to a specific voltage. It’s really not suprising. If it allowed you to determine the discharge current, it would probably explode, go on fire or fail…probably all three simultaneously. It’s the same as a bass audio speaker - needs to move a big amount of air slowly, so requires a huge magnet often with heat sinks as opposed to a tweeter which moves a small amount of air quickly and using a small magnet.

[Simon Chaddock]

Agreed but most chargers have an internal maximum on how fast you can set a discharge rate. As you say it will be limited by the components and the heat dissipation capacity.

Personally I prefer to "balance" discharge even if it takes some time so all cells are brought down to exactly the same voltage.  

[Witterings]

2 hours ago, Simon Chaddock said:

Agreed but most chargers have an internal maximum on how fast you can set a discharge rate. As you say it will be limited by the components and the heat dissipation capacity.

Personally I prefer to "balance" discharge even if it takes some time so all cells are brought down to exactly the same voltage.  

 

You're right they do but as I posted above, discharging creates heat and that's one thing electronics don't like, to me it's putting unnecessary stress on a fairly expensive piece of equipment which personally I'd rather not do and in the past have seen several comments generally advising against it.

 

The light bulb solution I posted a link to above does balance discharge, now I've found out I can just take a bulb or 2 out to reduce the amperage for small batteries, I'll probably stick with that for the time being.

If I do get the ISDT discharger in the future like you I'd rather balance discharge and I have a couple of LiPo balancers from before they were commonly built into chargers and I'm sure i could put one of those in the chain.

 

A big thank you to everyone for all their help and input, it is appreciated 👍 😃

 

 

[Nigel R]

Probably late to this but...

 

On 17/01/2025 at 13:19, Witterings said:

You're right they do but as I posted above, discharging creates heat and that's one thing electronics don't like, to me it's putting unnecessary stress on a fairly expensive piece of equipment which personally I'd rather not do and in the past have seen several comments generally advising against it.

Discharging via an external load won't stress the charger much differently to charging. It's simply switching a load on / off.

 

Internally, well, the case dissipates heat, the fan aids it, and the charger monitors temperature. Discharge rate will be set to keep things under control. Without knowing more I'd trust the manufacturer to limit this to something sensible. My bigger charger limits it to 20W (despite being a 300W charger) which doesn't take a very big fan or heatsink to control. You could always point an external fan at the unit if you're worried.

 

 

Edited January 20 by Nigel R

[John Lee]

On 17/01/2025 at 13:19, Witterings said:

 

If I do get the ISDT discharger in the future like you I'd rather balance discharge and I have a couple of LiPo balancers from before they were commonly built into chargers and I'm sure i could put one of those in the chain.

 

 

As an alternative to this you may want to consider one of the SkyRC dedicated discharger/analysers which when linked to an associated charger balance discharges. I have the BD380 & D200Neo combination which can balance discharge anywhere between 0.3A to 40A to either storage levels or another chosen voltage. 

[Witterings]

3 hours ago, Nigel R said:

Internally, well, the case dissipates heat, the fan aids it, and the charger monitors temperature. Discharge rate will be set to keep things under control. Without knowing more I'd trust the manufacturer to limit this to something sensible. My bigger charger limits it to 20W (despite being a 300W charger) which doesn't take a very big fan or heatsink to control. You could always point an external fan at the unit if you're worried.

 

I do get that, mine will charge at 1000 watts and discharge at 80 watts ... as I say though I've seen quite a few posts in the past advising against it when possible.

 

2 hours ago, John Lee said:

As an alternative to this you may want to consider one of the SkyRC dedicated discharger/analysers which when linked to an associated charger balance discharges. I have the BD380 & D200Neo combination which can balance discharge anywhere between 0.3A to 40A to either storage levels or another chosen voltage. 

 

I found the SKY RC ones afterward my initial post, the one down from yours the BD 280, I'll continue with the "bulb rig"I have for now but if I continue flying regularly again will definitely get one of those.