Add an extra cell to a nimh pack

[Andy J]

Have added an extra identical battery cell type to a 4.8v 3700mah nimh pack. Given the new cell and the existing cells will be a different states of charge am I better discharging the new 6v battery config before recharging to a full charge.

[Paul De Tourtoulon]

No just put it on a slow 12h charge the will all level out in the end, the 'overcharged' ones with throw the extra charge off in getting warm.

[Andy J]

Thanks Paul.

Edited November 22, 2022 by Andy Joyce

[MattyB]

This seems a slightly strange thing to do if the existing 4-cell pack was well used. What are the states of charge of the 4 cells in the original pack vs the new cell? If it were mine I think I would have tried to get everything within 0.1V of each other at the outset before doing any soldering, but @Paul De Tourtoulon is correct that if you charge slow enough it shouldn't do any major damage to treat them as he suggests. 

[Andy J]

MattyB, the 4.8v  pack was virtually brand new with few power cycles before I converted it to a 6v pack  so don't think there will be an issue with cell age.

 Unfortunately was unable to measure the cell voltages before the conversion but do suspect that there would have been a big difference in voltages given the new cell would have been shipped at a low charge status and the 4.8v pack was I think at a high charge status.

The new battery config did seem happy when I put it on a trickle charge yesterday with no cell getting warm so all seems OK.

Will put it though a discharge cycle today if I get a chance with a further trickle charge  just to settle in the new cell.

For further info the new cell was an identical brand type and power rating to the original pack so there should not be any issues on that front.

 

[Paul De Tourtoulon]

Don't worry about the number of cycles, when we first raced cars we threw in any battery's, only later on did we use 'selected' battery's with the

same internal resistances and we made up these to eke out the last drop of electric.

 

 remember these ?.

[Phil Green]

21 hours ago, Andy Joyce said:

Have added an extra identical battery cell type to a 4.8v 3700mah nimh pack. 

Curious as to why Andy?   🙂

[Andy J]

Had the 4.8V battery in a large model I was working up to be flight ready. Battery was on of two identical types feeding independent switches which then connected to an FRSKy dual power distribution unit which serves to monitor both input voltage from each pack as well as the output currents.

After only a short period of powered application found I was gettering telemetry warnings that the current draw was excessive. Therefore lost confidence that a 4.8V set up was compatible with the set up running 8 servos. Suspected the lower operating voltage and the resulting higher currents was the reason for the telemetry alarms so fitted a 6v battery as the primary power source and the second 4.8V as a secondary. Since doing that I have had no further issues although the model has yet to be flown.

 

That left me with a spare 4.8V battery and the need to fit a flight battery in a 1/4 scale Tiger Moth which I am also working on. Given the experience of high currents on a 8 servo set up thought it best to increase the pack voltage rather than expending more money purchasing a new 6V pack to drive a 5 servo set up in the TM.

[Andy J]

As a secondary issue to battery protection, can one purchase a proprietary foam to protect the battery from vibration?

[Nigel R]

https://www.ebay.co.uk/itm/264216906224?

 

All of my foam comes from stuff that arrives in the post. I save the thin stuff that looks like the above roll of wrapping foam.

 

What sort of servos? What was/is the large model? Just interested.

 

 

[Cuban8]

On 23/11/2022 at 16:12, Andy Joyce said:

Had the 4.8V battery in a large model I was working up to be flight ready. Battery was on of two identical types feeding independent switches which then connected to an FRSKy dual power distribution unit which serves to monitor both input voltage from each pack as well as the output currents.

After only a short period of powered application found I was gettering telemetry warnings that the current draw was excessive. Therefore lost confidence that a 4.8V set up was compatible with the set up running 8 servos. Suspected the lower operating voltage and the resulting higher currents was the reason for the telemetry alarms so fitted a 6v battery as the primary power source and the second 4.8V as a secondary. Since doing that I have had no further issues although the model has yet to be flown.

 

That left me with a spare 4.8V battery and the need to fit a flight battery in a 1/4 scale Tiger Moth which I am also working on. Given the experience of high currents on a 8 servo set up thought it best to increase the pack voltage rather than expending more money purchasing a new 6V pack to drive a 5 servo set up in the TM.

Something to be aware of.........simply adding another cell to raise the pack voltage to stop it sagging under load is not a good solution. Without getting overly technical, your pack simply needs the ability to source sufficient current without its voltage collapsing. Imagine running five AAA cells and expecting them to maintain 6V when under a heavier load than their chemisty will permit. Would you expect a car battery that's designed to start a mini to turn over the V8 in a huge pickup? Running servos on a higher voltage also increases their power and torque and ups their current need which will be self defeating.  If you need more current from a battery system, generally speaking use a larger capacity battery (some pitfalls with this) or parallel the batteries - how you get your car started with jump leads when you have a flat or weak battery.

[MattyB]

Was about to post exactly this, adding an extra cell in this instance will be self defeating - you need more capacity or lower IR to increase the ability to deliver the required current.

[PatMc]

What type & size (i.e. AA, C, D etc ) are the cells ?

[Phil Green]

Matty & C8 are absolutely right Andy.  Heed their advice 🙂

 

 

 

[Andy J]

MattyB and Cuban 8 obviously you did not read my post at 16:12 yesterday. 

To repeat the 4.8v 3700mah battery was indeed replaced with a 6.0v pack. What I probably should have said is the new pack is a sub C 5000mah.

However do concur that the two packs probably have a similar max supply current given the nature of nimh battery chemistry and both being sub C''s. 

Raising voltage will reduce current draw otherwise ohms law would be invalid on a constant load or fixed impedance.

The reason for the telemetry high current loads when using the 4.8v pack could have of course be due to the alarm settings which must be set on the Frsky RB-20 as these were certainly not set in the Tx.

Possible that the RB-20 likes a higher operating voltage but it''s operating voltage does say 4.8v to 8.4v.

 

 

 

[PatMc]

16 minutes ago, Andy Joyce said:

 

Raising voltage will reduce current draw otherwise ohms law would be invalid on a constant load or fixed impedance.

 

Ehh, no it's the opposite.

Ohms law states that I=V/R  

[Cuban8]

I know electrical  theory, even the simplest, can be a tad confusing but we really are talking about very basic stuff here and it should be familiar to every modeller. I don't blame those that shy away from the basic theory but if you do find things confusing it's worth getting the grips with the basics. It's not maths at such a basic level but applied arithmetic as used in a variety of practical situations that you wouldn't otherwise give a second thought to. The magic smoke is always looking for an easy way out.............it made me laugh at the time but I had a clubmate who wired up some LEDs on his model but omitted to consider the need for current limiting resistors. He'd previously fitted some flashing LEDs that could be directly connected to a power source and assumed all LEDs were the same.

 

Edited November 24, 2022 by Cuban8

[Martin Harris - Moderator]

Possibly some confusion from the common practice of producing similar power from higher cell counts in EP models. In this case, smaller propellers on higher cell counts are used to reduce current but as wattage (power) is similar and voltage is higher the result is that a lower current is used to produce the power.  
 

Using a higher voltage for servos will increase their speed and power but as stated by earlier posters, as the resistance is essentially the same, current will increase. 

[Cuban8]

A good basic tutorial here.

https://www.youtube.com/watch?v=uXr4lXYjXuU

[Nigel R]

22 hours ago, Nigel R said:

What sort of servos? What was/is the large model?

Think we may be missing the root cause of the problem. Can't see an answer to those questions yet. Further questions spring to mine also:

 

Why are the servos pulling a high current?

 

Do we know what current they are pulling? 

 

Lastly, was the model in flight or are we talking about bench setup?

Edited November 25, 2022 by Nigel R

[Andy J]

Well this is all getting very embarrassing. Had got it into my head that increasing the voltage would solve the issue with the over current telemetry alarms.  Will get the model back on the bench and investigate what is causing the issue before it ever gets flown.

Should be easy  looking at the difference between the 4.8v and 6v supply packs as the model now has both types fitted, both switched independently.  Establishing which channel is causing the alarm will be more difficult unless I can get my head around  the telemetry feed from the RB-20.  Can only be one of five output channels as the problem was first seen with the model on the bench with wings off.  Will disconnect the push rods on the elevator (2 off), rudder and motor just to check to check if any are binding. Cant believe it is the motor kill switch so will ignore that in the initial investigation.

[Engine Doctor]

A binding  linkage causing a stalled servo is a prime suspect for high current draw.

[Nigel R]

Aha. Bench, wing off, now we're getting somewhere 🙂

 

Agreed with ED. Might well be the kill switch causing one servo to bump against the end of switch travel.

 

I'd ask again what type of servos are fitted - an S133 has much less stall current than a hi torque spicy digital effort, for instance.

 

2 hours ago, Andy Joyce said:

Had got it into my head that increasing the voltage would solve the issue with the over current telemetry alarms

 

Which is why I asked whether problem was seen in the air against a regular flight load, or on the bench. In air loads tend to need a fixed amount of power to overcome, which would lead to a reduced current demand with higher voltage. Whereas, a problem seen on the bench might well indicate a different situation where perhaps even 'no-load' current was enough to cause it, and then the reverse would apply. Then we're thinking perhaps a dicky servo, or binding controls, etc etc.

 

More details = more useful / specific answers 🙂

[Andy J]

Servos are 2 x Savox SC0252 on the elevator, a Savox SC0251 on the Rudder and a Hitec HS645MG on the throttle.

 

None the wiser though what is causing the issue of over-current alarms.  Using the 4.8V flight pack which initially was giving an indicated 5.0v on the RxBt channel with a current drain of 0.35A. Every now and again (possibly once every 3-5mins) associated with a servo movement would generate a single overcurrent alarm but not on subsequent control inputs.

After some time when the RxBt dropped to 4.8v at random intervals an overcurrent alarm was issued even without a control demand.  Following this alarm once again subsequent control movement demands did not generate the alarm. It is thus seems that the RB-20 appears to be generating random alarms for some reason when supplied by the 4.8v 3700mah pack.

Interestingly with the 6V pack applied the RxBt voltage gave 6.6v indicated with a lower current drain of 0.18A.  On no occasion did the RB-20 generate an alarm when using this supply.

 

 

[Nigel R]

Thanks Andy. Think we're getting closer. Despite those being quite beefy servos there, all 10k to 15kg sort of size, a Sub C pack should be untroubled by them. The indicated 4.8V is rather low for a fresh pack. Sub C should be capable of 10A or so before even coming close to that terminal voltage. Maybe the RB-20 is dropping some volts to perform the backup function? I'm speculating.

The RB-20 seems to use the same alarm to indicate low voltage, as well as high current. The manual says it has an alarm for every servo channel - maybe there is only one problem servo / channel here? Does the "RBCS" telemetry field show anything odd? (normal being "OK").

 

By the way - is the 350mA current at idle - or when the sticks were moved?