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Starting battery for multi glow motors?


Martin Fraser
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Hi folks,

 

Since the demise of the glow driver on my field box panel, I've been using a single JP 2v Pb battery to start my Saito twin. It seems to cope lighting the two plugs for the day's flying. However, using it to start my new 3 pot radial on its test stand it struggles after one or two attempts and the voltage plummets from 2.05v to 1.8v. What's the best plan for this engine - an on board glow driver or a larger capacity 2v Pb battery or a couple of them in parallel?

 

TIA and best wishes for 2022.

Cheers, Marty  

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I use a 6600mah nimh pack of 4 sub c cells (2 series 2 parallel) for 2.4v. This setup is man enough for my OS pegasus with 4 plugs to cook and i also use it on my various twin cylinder engines with the plugs wired up in series. 

 

With 3 cylinders you would probably have to wire 2 in parallel and grab the 3rd in series as the ground. 

 

The alternative would be to use your current 2v for 2 pots and have another glow driver for the remaining cylinder. How accessible will the engine be once installed?

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As the plugs are powered in parallel I'm surprised that 1.8v is not enough to make them glow sufficiently, or is that the voltage measured off load after a couple of attempts, I start my laser V twin with a 2v pb cell with the plugs in series and it starts fine. If it's the voltage off load it sounds like you cell is struggling to provide the amps when it's underload.

 

So either 2 of them in parallel or a couple of Sub C Nimhs in parallel should do the job.

 

(John just beat me to the post ? )

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4 minutes ago, Jon - Laser Engines said:

With 3 cylinders you would probably have to wire 2 in parallel and grab the 3rd in series as the ground. 

 

 

Don't think this is a good idea as the 3rd plug would have to handle twice the current of the 2 plugs in parallel, so you'd either risk blowing this plug or the first two may not glow bright enough.

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i was thinking about that Frank but the current through the lone plug would be no more than it would see if you ran only 2 plugs in series and it handles that just fine. When setting up my pegasus i could only get 3 cylinders going and found a bad connection on one glow wire so i had this 2p 1s setup and nothing died. Also as the setup is beefy enough to fire 4 plugs (2s 2p) there should be no issues with dim glows on the parallel side. 

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Hi gents,

 

Jon - it's going into the Swordfish (one day) so will be pretty inaccessible.

 

Frank - The voltages are off load so I think my JP battery is on the way out. Shame as it's only a couple of seasons old. However, this maybe down to me charging it at too high an amp.  I now set the charger to Pb, 2v @ 0.2amps.

 

Many thanks for the suggestions,

Marty

 

 

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3 hours ago, Jon - Laser Engines said:

i was thinking about that Frank but the current through the lone plug would be no more than it would see if you ran only 2 plugs in series and it handles that just fine. When setting up my pegasus i could only get 3 cylinders going and found a bad connection on one glow wire so i had this 2p 1s setup and nothing died. Also as the setup is beefy enough to fire 4 plugs (2s 2p) there should be no issues with dim glows on the parallel side. 

John, it's possible it worked OK, but the current is split between the two in parallel , basically if you treat them as resistors, 2 resistors in parallel have half the resistance as a single one and as V=IR the voltage drop over the first two parallel resistors will be a 1/3rd of the single resistor. Not an exact analogy as the resistance of a glow plug varies,  but you'd have two plugs glowing duller and one glowing brighter, may not be a problem but it could be, depending on how well your battery holds up under load.

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Why not simply wire all the glow plugs in parallel?  Assuming a decent battery with reasonably low source resistance, it will have a constant terminal voltage, so each plug will see the same voltage (and hence draw the same current) as it would if it were fitted to a single cylinder engine.

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Yep, but the battery will need to be capable of delivering around 4 amps, if the cell is tired it may not be capable of this.

 

2 Sub C 3000+ mah cells in parallel will not only be easily able to do this at the correct voltage but will also have a bit more capacity than a 2v 4.5ah cell. Other advantage is that you can charge the Nimhs at 1c but pb is best at 1/10th c 

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3 hours ago, Frank Skilbeck said:

Yep, but the battery will need to be capable of delivering around 4 amps, if the cell is tired it may not be capable of this.

 

2 Sub C 3000+ mah cells in parallel will not only be easily able to do this at the correct voltage but will also have a bit more capacity than a 2v 4.5ah cell. Other advantage is that you can charge the Nimhs at 1c but pb is best at 1/10th c 

Yes, you’d need sufficient capacity to deliver the current. Charging NiXX batteries in parallel is supposed to be a no-no so you’d need to bear this in mind when assembling the system. 
 

P.S. According to the makers, a 2.5 Ah D Cyclon cell is capable of 65A discharge!

Edited by Martin Harris - Moderator
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I used to use 3 x 2,5AH Cyclon batteries in series fitted in a bidon to power my bike lights for my winter commutes.  When I started aeromodelling after I retired I used one to power the glow plug on my trainer and never had a problem.  They are incredibly powerful with a very low source resistance as Martin writes.  I used to charge my bike battery at work and commandeered a bench power supply for the purpose.  IIRC the 3 in series charged quickly with a constant 6.5v supply and lasted many winters

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2 hours ago, Martin Harris - Moderator said:

Charging NiXX batteries in parallel is supposed to be a no-no  assembling the system. 

 

my 2s 2p glow battery is a sealed pack. Seems to charge ok though ? Do we know why its considered a bad idea?

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Quick Google:

 

"NiCad and NiMH batteries are amongst the hardest batteries to charge. Whereas with lithium ion and lead acid batteries you can control overcharge by just setting a maximum charge voltage, the nickel based batteries don't have a "float charge" voltage. So the charging is based on forcing current through the battery. The voltage to do this is not fixed in stone like it is for the other batteries.

This makes these cells and batteries especially difficult to charge in parallel. This is because you can't be sure that each cell or pack is the same impedance (or resistance), and so some will take more current than others even when they are full. This means that you need to use a separate charging circuit for each string in a parallel pack, or balance the current in some other way, for example by using resistors of such a resistance that it will dominate the current control."

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26 minutes ago, Jon - Laser Engines said:

Do we know why its considered a bad idea?

 

You've seen the graphs of nimh terminal voltage during charging, I imagine:

 

spacer.png

 

Imagine you have a perfectly fully charged nimh (call it A) at 100% charge, connected in parallel with an accidentally slightly overcharged nimh (call it B).

 

The overcharged nimh (B) has a lower terminal voltage. To whit, that means that cell B will start accepting charge from the perfectly charged nimh (A). i.e. (B) will continue to become even more overcharged, as A is discharged. Overcharge is obviously a bad thing.

 

A parallel pair also make a flatter peak, so the usual peak detect thing is harder pushed to figure out when charging is complete. Plus it's guaranteed that the peak will be under where one cell needs it, but over where the other needs it.

 

Instead of parallel, the right choice is a bigger cell.

 

Or, alternatively, mess about with some sort of protection device(s) to prevent this failure mode.

 

Edited by Nigel R
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interesting stuff. My 2 delta peak chargers handle the 2s 2p pack (2.4v 6600mah) fine but both are less convinced by my 2 cell parallel packs (1.2v 6600mah) and i have found them trying to stuff 8000mah into the 6600mah packs. All of these packs came pre assembled with on board glow units i bought and there is no other way of charging them. 

 

In the case of the twin cell packs i used to turn them off after a given time, the 2s2p pack peaks fine so just leave it alone. My charger is set with a 6500mah cut off though and the pack is 6600mah. Normally it peaks at around 5800-6200 even when stone dead flat, which it normally is when it comes to charge it as i only charge it when my glow packs up. As i dont use the OBG systems the 1.2v packs came with i wired up a harness to turn them into a 2.4v 2s2p pack as a backup for the other. Now my charger seems to DP charge them quite happily so i again just leave them to it. 

 

Its probably another example of it not being the perfect setup but working well enough for the purposes of powering a glow plug or 4 while i drag the packs around in the mud. 

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Morning all,

Some interesting points folks.

Following John's idea of a 3 plug test rig, I connected my fully charged Pb cell (2.01v) and all three plugs lit up nicely - for about 30 seconds and then they died. The voltage collapsed to 0.5v and slowly recovered to 1.5v once off load. For simplicity, I might try a Cyclone cell as the JP batteries seem very thin on the ground. It should be fine on the twin pot and hopefully the 3.

Cheers, Marty 

 

 

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10 minutes ago, Martin Fraser said:

I connected my fully charged Pb cell (2.01v)

Hi Martin,

 

A 2V lead acid cell should be about 2.12V when fully charged. If yours only went up to 2.01V then either the cell is suspect or the charger isn't supplying the correct voltage to fully charge it.

 

You previously mentioned that you set your charger to 2V 0.2A, are they maximum values that the battery will see during charge?

 

Brian.

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55 minutes ago, Martin Fraser said:

Morning all,

Some interesting points folks.

Following John's idea of a 3 plug test rig, I connected my fully charged Pb cell (2.01v) and all three plugs lit up nicely - for about 30 seconds and then they died. The voltage collapsed to 0.5v and slowly recovered to 1.5v once off load. For simplicity, I might try a Cyclone cell as the JP batteries seem very thin on the ground. It should be fine on the twin pot and hopefully the 3.

Cheers, Marty 

 

 

 

Did you measure the current draw?   Cyclon batteries are very good but the last time I looked (some years ago) they were quite expensive.  The 2.5AH ones are roughly the same size as D cells IIRC.

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1 hour ago, RottenRow said:

Hi Martin,

 

A 2V lead acid cell should be about 2.12V when fully charged. If yours only went up to 2.01V then either the cell is suspect or the charger isn't supplying the correct voltage to fully charge it.

 

You previously mentioned that you set your charger to 2V 0.2A, are they maximum values that the battery will see during charge?

 

Brian.

Hi Brian,

Yes that's what I've been setting the charger to lately and it used to go up to 2.12 ish when fully charged but barely makes 2v now. I think I may have damaged it by inadvertently last year with the wrong voltage or c rate.  To be honest, I'm not sure what a 4.5amp cell should be charged at.

Cheers, Marty. 

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42 minutes ago, Geoff S said:

 

Did you measure the current draw?   Cyclon batteries are very good but the last time I looked (some years ago) they were quite expensive.  The 2.5AH ones are roughly the same size as D cells IIRC.

Hi Geoff,

No and I'm not sure how to do that?.  Yeah they're not cheep but having just bought a Saito radial I shouldn't complain too much!

If I look after it, it should do a few seasons.

Cheers, Marty

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