discarging nicd and nimh how?

[snakeyme 0]

i do have a futaba battery pack but everytime i charge the pack (or try to) it gets so damn hot (i use a charger with auto cut off to stop over charging) and by hot i mean to hot to touch so im too affraid to give it a full proper charge as it gets this hot within an hour of charge.

 

This is the packs info and charger info:

 

Battery;

Futaba p-pack NR-4J

4.8VDC - 500 mAh

Charging: 50mA - 15 hours

Quick CHG: 150mA - 5 hours.

 

Charger;

Ripmax

Pri: 110-240v ~50/60hz

Sec: Rx 4.8-8.4v - 600mA

Tx 9.6v - 120mA

[Martin 0]

Capacity is measured in milli-amp hour (mAh) which indicates how many constant milli-amps of current can be supplied by the pack for one hour.

 

Actually, most (all?) batteries have their rated capacities based on 20 hours rather than 1 hour. Therefore, a 2000mAh battery is actually rated to deliver 100mA for 20 hours, not 2A for 1 hour. If you try the latter you will probably see a significantly smaller usable capacity. Compare the graphs I posted earlier, which only go up to 1A.

 

(This is particularly crazy for things like car batteries, where (say) a 40Ah capacity is still based on the idea of drawing 2A for 20 hours, despite the fact that the battery is really designed to deliver hundreds of amps for a few seconds.)

 

However, this is just me being pedantic, and isn't really relevant to the 4.8/6V question!

[Martin 0]

i do have a futaba battery pack but everytime i charge the pack (or try to) it gets so damn hot (i use a charger with auto cut off to stop over charging) and by hot i mean to hot to touch so im too affraid to give it a full proper charge as it gets this hot within an hour of charge.

 

This is the packs info and charger info:

 

Battery;

Futaba p-pack NR-4J

4.8VDC - 500 mAh

Charging: 50mA - 15 hours

Quick CHG: 150mA - 5 hours.

 

Charger;

Ripmax

Pri: 110-240v ~50/60hz

Sec: Rx 4.8-8.4v - 600mA

Tx 9.6v - 120mA

 

You have a big mismatch between battery and charger.

 

Your battery has quite a small capacity of 500mAh, and the recommended charge rates are 50mAh (0.1C) or quick charge at 0.3C. These charge rates are pretty standard recommendations for both NiCd and NiMH.

 

Your charger is putting out 600mA, which is 1.2C, four times the recommended fast charge rate. That would be considered faster than normal even for a LiPo.

 

Chargers for NiCd and NiMH charge at constant current, which means the charger will adjust its output voltage as required to try to force the specified output current into the battery. (This is different from constant-voltage power supplies, which put out a fixed voltage and whose current rating is merely the maximum at which this voltage can be maintained reliably and without overheating or damage.) Your charger is trying to force 4x the recommended fast charge current into quite a small battery, and to do this will be winding the voltage up very high to overcome the battery resistance.

 

NiCd and NiMH batteries are designed to tolerate mild overcharging for a short period of time. In fact, this is the reason for the recommended standard charge rate of 0.1C - a fully-charged battery will convert excess energy into heat, and this low charge rate allows the battery to do this for several hours without ill effects. This is also why NiCd and NiMH batteries do not require balancing like lithium packs do - because all cells in the pack receive the same current, even when a few cells reach capacity early, they will just start to warm up while the others finish their charge. At fast charge rates (higher than 0.1C), the battery is not able to convert the energy into heat safely. Fast chargers should always have some sort of intelligent cut-off to detect the pack becoming fully charged and stop charging.

 

I would be quite suspicious of that pack now. When NiMH cells are badly overcharged they produce gas internally, and to avoid the cells exploding there is a pressure release valve. Unfortunately, once the gas has escaped the battery will not work properly.

 

I have one of those same chargers (or one very similar). I think the RX output is really designed for higher capacity packs - possibly for a fast 0.3C charge of a 2000mAh pack, or for a standard 0.1C charge of a 6000mAh pack. I suspect it's really designed for NiMH-powered electric RC cars or similar, where you would expect to see reasonably large capacities in packs constructed from C cells.

 

I would highly recommend a decent CPU-controlled charger/discharger, which would start at around £30-40. This would allow you to cycle your packs to maintain them in top condition, and also measure their capacity to reassure yourself they're still OK (after a long period in storage, or being over-discharged, for example). Also, if you ever want to go electric then it should do LiPo charging too.

 

If you really want to do this on the tightest budget, then there are dumb NiMH chargers available which do have selectable output current, for example the first one on this page:

http://www.component-shop.co.uk/html/ra ... trol1.html

If you use a dumb charger like this, I would only ever charge at 0.1C or lower, and do check the battery temperature while it's charging. You might not want to charge the battery in your model, for example.

[snakeyme 0]

thanx martin, i am reluctant to use it for actual flying and only use it for testing the installed servos within the plane when im at home and setting up the plane before taking it out for a flight otherwise i dont bother with it and if ur wondering how long i charge it to do the testing, never more than 5 mins ( usually 1 or 2 min).

 

But if u think i should just bin it, then please say so

[Lofty 5]

Just my thoughts !

 

I stop using NiCd's & NiMH where I could along time ago! I know they have been around a long time and are / where the standard batterys for RC for a long time! But I've just never been happy with them for a lot of reason's!

 

I moved over to Reg's and 2Cell Li-Pos at first, for most of my plane's! But always thought the Reg. could let me down.....

 

So now I am using Lithium Phosphate Packs "Li-Fe, A123, Li-FePo4....or what ever they call them !"

The only three down side's atm, are cost, size "mAh" and you need a newish charger!

[arun 0]

But if u think i should just bin it, then please say so

 

Maybe that is your decision to make based on the (very comprehensive) facts presented!

 

Ultimately this hobby is a form of engineering - a willingness to analyse the facts for yourself be they based on electronics, mechanics or aerodynamics and make a pragmatic decision will serve you well.

 

None of these things are essential to succeed, but they damn well help

[snakeyme 0]

very true arun, trouble is that sometimes i cant see the obvious because of different reasons, may it be budgeting, being narrow minded on certain things or becoz of lack of experience, as for using the battery pack for in-house testing and setting up, i cant really see a problem with it aslong as i dont leave it in the plane but others may say its a huge risk, especailly around a 1 year old (btw, i never start an engine anywhere near him before anyone does ask).

 

So i guess that you could say that the lack of experience with this is my akeles heel, even though im experienced with electrics.

[stalion456 0]

Just my thoughts !

 

I stop using NiCd's & NiMH where I could along time ago! I know they have been around a long time and are / where the standard batterys for RC for a long time! But I've just never been happy with them for a lot of reason's!

 

I moved over to Reg's and 2Cell Li-Pos at first, for most of my plane's! But always thought the Reg. could let me down.....

 

So now I am using Lithium Phosphate Packs "Li-Fe, A123, Li-FePo4....or what ever they call them !"

The only three down side's atm, are cost, size "mAh" and you need a newish charger!

 

 

What about voltage/ life are 6.6 v how do you make it work or bring it down to 4.8v?

[Martin 0]

thanx martin, i am reluctant to use it for actual flying and only use it for testing the installed servos within the plane when im at home and setting up the plane before taking it out for a flight otherwise i dont bother with it and if ur wondering how long i charge it to do the testing, never more than 5 mins ( usually 1 or 2 min).

 

But if u think i should just bin it, then please say so

 

That battery and that charger are horribly mismatched, causing your battery to overheat when charging.

 

As a result, you may have caused permanent damage to the battery. I wouldn't use it for anything safety critical like a flight pack, at least not without thoroughly testing the battery out (at minimum, a good few charge/discharge cycles on a decent CPU-controlled charger, monitoring the capacity during discharge to see what the real battery capacity is).

 

For a pack basically used for ground-based servo testing it should be good enough, provided you can find a way to charge it safely - and there the problem is really with your charger, not with the pack. Even if you buy a new pack you will still need a new charger - at minimum, something that has a switchable output current.

[Martin 0]

What about voltage/ life are 6.6 v how do you make it work or bring it down to 4.8v?

 

You don't try to bring it down to 4.8V. If your electronics can run off a 5s NiMH pack (nominal voltage 6.0V, but fully charged voltage around 7.2V) then they should be OK on a 2s LiFe pack (nominal voltage 6.6V, fully charged also around 7.2V).

 

For example, see the discharge graphs (figure 2) on this page (I posted NiMH discharge graphs in an earlier post):

http://www.efirstpower.com/li.html

 

There is more information about this battery type here:

http://en.wikipedia.org/wiki/Lithium_ir ... te_battery

[stalion456 0]

Hi Martin,

 

I currently run all my servos and electronics on 4s 4.8v if I wanted to use life packs would I have to change the servos? the current servos are for a max of 6v, so what I meant is: in order to use life with the current set up that I have in all my planes is there any device/way of bringing the voltage down so that the servos will not suffer? or is a question of changing all the servos to work with a fully charged load of 7.2v from the life packs? because for someone like my self with 10 aircraft set up it would be very expensive to change all servos, maybe any new models could benefit from life packs but unless there is a way of bringing the voltage down it would be futile, like if you start on a brand of transmitter and buy matching receivers it's best to stick to what you have started with unless your starting new/or have spare cash.

 

Tha's what I meant in regards to lofty's comment.

[arun 0]

I've just ordered a couple of these to test, will report back when they turn up from China

 

http://www.hobbyking.com/hobbyking/stor ... duct=23823

[Bravedan 106]

Interesting and dirt cheap.

 

Is the UBEC, a device that can get quite hot usually, sticking out the rear on a carefully staged picture or not, and if not, how have they arranged cooling I wonder. Might seem a strange choice of a JST plug, but I'd guess they were trying to prevent people not paying due attention charging them in a model and sticking them on a more usual Rx battery charger. I suspect many will change it for a normal Rx plug anyway. (Some ACOMS and other low end sets use JST for NIMH!!)

 

I have been using UBECs and LiPos as Rx supplies for ages, at least four years, particularly in my smaller slope soarers where the weight was not needed for ballast anyway. My Panic uses one usually with a 800mAh 3S to keep it light and yet able to deliver a lot of power, never ever used more than half its capacity in a day.

 

Several of my bigger Helis use UBECs (with LiPo not LiFe) and OPTO ESCs where electric powered. UBEC use on Helis is probably more common than not due their extreme current draw, so its not a new thing to a Heli flyer, where split system voltages are also quite common.

 

I have though tried to stay away from the more uncommon/oddball batteries (at least for model use) like A123 and LiFe, as they seem yet another complication in use and another chance for getting it wrong. Also, only one of my four bigger chargers can deal with LiFe/LiFo anyway. May be a stupid choice to avoid them, but it is KISS principle.

 

Be a little circumspect, as having had top price ALIGN branded Regulators which did anything BUT regulate to their stated voltage, to be sure, don't trust the output in use until tested quiescent AND under load.

 

Be interested in how you get on but I don't think its a road I'll be treading.

[Bravedan 106]

i probably shoulda mentioned this before but i wrap the loose cell battery carriers in insulation tape so that only the contacts are visible and then put the whole thing in a tightly cut piece of foam (the stuff that usually pads fuel tanks) and its so tight that its a struggle to get it in, this is what i do to stop them coming loose

 

Think of the pack connections. Every friction connection adds a risk that it will fail, get dirty, go high resistance, or the cheap "spring" material will weaken with use and lose full contact (proven common, especially if the flat strip type).

 

A properly spot welded commercial cell pack has two points of friction contact, each pole of the plug.

 

A clip in 4xAA pack has those PLUS at least eight others.

 

THAT is where your increased risk is.

 

Wrapping the pack up will only really make it hotter while charging, possibly even making the generally very low quality "spring" more likely to lose tension, it will certainly not reduce that basic risk to any significant degree. Also, once its thoroughly wrapped you probably will never see if the connections are suffering corrosion, so you will have increased the likelihood of high temps during charging causing leakage to a cell, the corroding connection of which you will not see happening.

 

As Arun says, your choice!!!

[snakeyme 0]

how do u find out what current servos will or can i post what servos i got in each plane and you guys could let me know.

[Bravedan 106]

how do u find out what current servos will or can i post what servos i got in each plane and you guys could let me know.

 

That's actually a very good question to which sadly there isn't a very good answer.

 

For example, Futaba's own website servo tech detail table does not tell you. Neither does JR, nor Savox. They all concentrate on size, speed and torque. To be fair, it depends on so many variables like resisting forces, lever length, digital or analogue, brushless or coreless or brushed, voltage applied, so there is no single easy answer. They could give a stall current I suppose!

 

The only reliable way to prove current draw is to assemble your "set" and then insert an ammeter in the main feed from the battery, and go wiggle everything furiously , and even that won't simulate flight loading.

[arun 0]

That's actually a very good question to which sadly there isn't a very good answer.

 

Ah, but there is a good answer! One of the Safety Master Servo Testers in your plane temporarily to test in flight will tell you current over time as a graph. £21 is a pretty reasonable price for the peace of mind on a big plane.

 

[attachment=0]4460470a.jpg[/attachment]

[Martin 0]

Is the UBEC, a device that can get quite hot usually, sticking out the rear on a carefully staged picture or not, and if not, how have they arranged cooling I wonder.

 

Do you mean Arun's link? I don't think there is a UBEC in the picture. With 7V tolerant electronics LiFe can be used directly. Along with being reputedly the most stable of the lithium-based chemistries, that's one of its key advantages.

 

I have been using UBECs and LiPos as Rx supplies for ages, at least four years, particularly in my smaller slope soarers where the weight was not needed for ballast anyway. My Panic uses one usually with a 800mAh 3S to keep it light and yet able to deliver a lot of power, never ever used more than half its capacity in a day.

 

That's with a switched UBEC, I assume? I always balked at the idea of sticking in a LiPo and a linear BEC, as throwing away 30% or more of my battery energy (and producing yet more heat to deal with) before doing anything useful seems wrong. With a switched UBEC able to use the higher voltage to draw less current from the battery this does seem like an option, especially if you have an existing supply of the relevant batteries ready to be swapped between models - always nice to keep down the number of different parts required!

[snakeyme 0]

these are the servos that i have in my planes:

 

1.) hi-boy precedant;

A.) aerilons, elevator and throttle: futaba FP-S148.

B.) rudder and steering: futaba S3003

 

2.) topmodel kabriolin;

A.) elevators: hitec HS-77BB x2

B.) rudder: hitec HS-300

C.) throttle: futaba S3004

D.) aerilons: futaba S3003 x2

 

 

Could you also tell me if these servos are actually any good.

[arun 0]

Although we went off topic a while ago, it's probably time to either come back on topic about nimh batteries, or start a new thread for this question.

[Bravedan 106]

That's actually a very good question to which sadly there isn't a very good answer.

 

Ah, but there is a good answer! One of the Safety Master Servo Testers in your plane temporarily to test in flight will tell you current over time as a graph. £21 is a pretty reasonable price for the peace of mind on a big plane.

 

[attachment=0]4460470a.jpg[/attachment]

 

Had forgotten about that unit!

 

Reason is, I'd discounted it, as I probably only have a single model it would fit into, so not a good answer for me, as you say, big plane only.

 

I think they missed the point a bit with that one, OK, if you can fit it, it does give point of use standalone indication, but IMO they would have been better to have had the sensor recording unit separable in the lead and then plug that into the display part (and/or PC), then it would have fitted FAR more installations.

 

Having a high current but still very small unit for EP POWER measurements (DPR) which connects to a PC, it is possibly that sort of small and light unit design that would service the majority of installations. Sadly, don't know of one.

 

Written while just completing the weekly task of charging just under 40 Tx's plus spare batteries and over 80 model battery packs, to keep on topic.....