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adamjedgar

Nimh vs Nicad batteries for electric start

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Hi guys,

I recently decided to replace my Nirvana Rodeo 200cc engine Nicad battery pack with Nimh.

 

The old pack consists of 14 sub C batteries in series to produce 16.8 volts.

After making up the new pack and then charging batteries 1 week ago, I installed the new pack into motor yesterday and attepted to start ...it barely turns the Simonini 200 over.

Ideas?

Edited by adamjedgar

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The key to batteries for high current applications is to make sure the ones you have are up to the job. I would use Lipo. Both my Nitro and Tornado now run on the same ultra-lightweight packs that can manage 75 amp constant current....more than enough

https://hobbyking.com/en_us/turnigy-graphene-1000mah-6s-75c-lipo-pack-w-xt60.html

20190704_104714.thumb.jpg.da77d91bf346e1e00da583af3819bab5.jpg

 

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What voltage is that?

 

The Rodeo is 16.8v. I can't get this voltage with lithium...nearest is 18v which won't work because of the charging system on the Nirvana Rodeo. It's a 16.8V system

 

I have since read that Nimh aren't good for high current drain...such as electric starters

Edited by adamjedgar

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Don't use a Lithium Polymer battery with a charging system designed for nicads, you will have a battery fire.

Ive been using Lipo batteries for years in RC models, they require very specific charging conditions.

I have a Bailey 4 stroke which I use a 4 cell 2200mah Lipo as the starter battery, I have disabled the charging circuit and charge it when its removed from the motor. I can get over 40 starts from it between charges.

A 4 cell Lipo is 14.8V minimum and will start your motor without any problems.

Is that a possible solution for you?

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I am not sure Vauxscott...i have in the past turned it over using a 12volt motorcycle battery. The Nirvana fired up ok, however, if a starter motor normally runs on a 16.8 Volt system, will using a 12 Volt battery cause problems for the starter motor? 

 

I ask that because i once had a fridge at our house catch fire and burn half a house down because the fridge motor supply power cable was damaged and the compressor was unable to draw enough to run properly. Eventually it over heated and caused a fire. Would not the same thing happen to a starter motor that is not being supplied with enough voltage? (ie less than  the designed16.8V)

Edited by adamjedgar

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I'd be surprised if your starter is designed for 16.8v.   I would say it's more likely to be an automotive one designed to run on 12v that's just being over driven slightly.

 

 

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Most of these starters we use are based on scooter starters and have a range of 10v to 20v, as long as you stay within that range you'd be fine. Have a look at your starting relay, it will state a max current rating and possibly a voltage too, that will be your limiting factor.

With Lithium batteries the actual voltage of a charged pack can be a good bit more than the nominal voltage, but with 4 cells you are safely within the range.

Scott.

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The lower the voltage battery, the lower the current. However, usually going slightly higher on voltage helps as the machine turns over so quickly it starts immediately.

 

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7 minutes ago, AndyB said:

The lower the voltage battery, the lower the current. However, usually going slightly higher on voltage helps as the machine turns over so quickly it starts immediately.

 

 

If you're thinking Ohms law, it's the other way round.  Reduce the voltage and the current draw will increase.

 

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No, you are completely wrong.

V=IxR so I=V/R since the resistance of the windings is constant, the lower V, then the lower the I

This is why a starter works much better with higher voltage, because the current increases, hence the power increases.  BUT, you have to be careful not to burn out the windings using higher current.

 

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ps, power = IxIxR so the power goes up with the square of the current. In other words, a small increase in current = a much bigger increase in power.

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12 hours ago, adamjedgar said:

The Rodeo is 16.8v

I suspect NIMH packs see a MUCH higher voltage drop under load than a LiPo pack which are typically very good under high load and that will be why they have gone for 16.8v
Both systems may only output 9 or 10v during starting despite the vast gap in voltage when resting (similar to a lead acid starter battery drop)
I start my Baileys four stroke with a three cell 5ah LiPo pack (nominal voltage= 11.1v) I cut the charge wire and like Scott, I charge at home (not as inconvenient as it sounds)
If Scott is using a 4 cell, that would be 14.8v nominal on the same motor I am starting with 3 cells (11.1v) so there is obviously some wiggle room :) 

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This is why Lipo's have a "C" rating. This is the continuous current that the pack will maintain without dropping voltage. Lipo's are fantastic at delivering high load right up to the point they run out of capacity.

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Ok...my preference would be to go for Lipo...I have seen those in model aircraft and are great.

 

Concerns...

1. On the nirvana the battery pack is built into the fibreglass shell in a pocket right behind my back. Are they safe enough to be in such a location? (Fire)

2. The system charges to 16.8volts (nicad) which peaks I believe at 22volts. I would need to change this...I'm thinking it may be better to drop back to 12 volt motorcycle lipo. Is it easily doable to change the nirvana rodeo (simonini200) over to 12 volt and then I can use a standard motorcycle 12volt Lipo battery? What is involved in this?

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I do understand why you would like to remain self contained with the charge system.
I bought a charge board to do just this with my LiPo but I smoked that board on first startup and have not yet got to the bottom of why (it may need a capacitor after the regulator to keep the voltage stable or I may have just missed a short...?)

LiPo batteries are fussy about being well treated and require a dedicated charge regime (including balancing)
You may be able to find a charge board that is ok taking 22v input (mine can only take 15v max which may be why it smoked without the cap to tame wild voltages)

A dedicated motorcycle LiPo will be very costly, bulky, a shit shape and require a new magneto on your motor... BUT will likely have charge circuitry built in.
A 3 cell 11.1v 35C 5000mah lipo will cost you £27 (a 4cell 14.8v will be closer to £50)
B6 charger less than £15

I am sure that is why most seem to charge separately when they convert to LiPo… it saves all the dicking about.
Buy a battery and charger, solder some new connections, cut the charge wire, job done.

My LiPo is mounted behind my back and it really ought to be elsewhere or be in a fireproof bag... its not wise to mount it where it can burn through your harness even if its being well looked after.

You have some research to do and some decisions to make :( 

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Thank's guys for the input. I have some thinking and tinkering to do.

I guess the short of this, I am dismayed at my Nimh failure...I really thought that was going to be a winner cause everyone say they are better. I missed the part where Nimh are apparently crap for high drain applications.

 

Anyway, I have one more avenue of trial and error with Nimh, I am going to use heavier gauge metal (copper that was previously on my old nicads)  linking batteries together in series. This is just to make sure I am truly comparing then properly. If the Nimh batteries still won't crank the motor, then I will know for sure it's a fail. I am persisting just a little longer because Wikipedia says these batteries should work (at least that my comprehension of the Wikipedia writeup)

 

If the Nimh turn out to be a fail, in the short term I will have to use Nicad until I can convert the whole thing to 12v. Once it's 12v, I can use the battery my other paramator has (which is a compact lightweight lithium motorcycle battery and is brilliant).

Edited by adamjedgar

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well i have managed to finally finish soldering up the Nimh battery pack with heavier duty copper pieces for linking batteries.

i put it into paramotor this afternoon and unfortunately it was a complete fail. There is barely enough cranking amps to turn the 200cc motor over.

I have to say i am disappointed, i was sure that this would work...even a local battery supplier (who also make up battery packs) were certain that the Nimh batteries would be far better than Nicads.

It would seem that the few forum posts i have managed to google comparing Nicads and Nimh batteries are correct...for electric starter cranking power, Nimh are pretty much useless.

Back to the drawing board...i guess tomorrow i will be heading out to buy some Nicads to make another battery pack up. I will post back once i have done that...might even make a video about it as I am sure a real life comparison might be of use to others.

Edited by adamjedgar

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After using my 1000 mah Lipo pack (yes, a tiny little one) for the past month I thought I had better charge it. It was only down to 65%.

I use 1000 mah, 75 c rating (so will do 75 amps continuous), 6 cell (so 22 volts).

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Not directly related to this thread, but on a 'similar' line from someone who has NIL understanding of electricity.

I start my trike mounted Rotax 503 with electric start, but to keep the battery size to an acceptable size and weight it is only 4 amps. This just about turns the motor when cold so to give a bit of ooomph when starting for the first time I add a jump start with a 7 amp LiPo. Th2 combinedd turns the motor really well so that I can warm it all up nicely.  Then after attaching the trike to the wing (the engine still being warm) the 4 amp battery is quite sufficient. BUT, given that the engine is charging the battery while it is running, would I be able to fit the LiPo directly to the charging system of the engine and do away with the jump start?

I've kept away from this to date as I have no wish to start a fire at any time, never mind in the air. I have been told that a LiPo can be charged this way but obviously it isn't a balanced charging system when charged by the engine.

Any ideas?

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Do not ever charge a Lipo without using a proper "balance charger". Never charger them without them being in a fire proof bag. Lipos are safe provided you follow the rules. 

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33 minutes ago, Guy said:

I have been told that a LiPo can be charged this way but obviously it isn't a balanced charging system when charged by the engine.

I have also heard of people who forgo the balance cycle for long periods without any major departures in balance but a magneto / regulator system would make me nervous even if I had a plot of the output under all conditions.
Even if the voltage never exceeded the nominal pack voltage you would need to be sure that your system couldn't exceed the packs charge current limitations.
IF I was going to experiment, I would want that pack mounted in such a way that if it went off like a hand grenade, it couldn't do anything but cost me a new pack :)
 

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The problem with not balance charging Lipos is that when one cell partially fails (only takes a small voltage) the other cells have to take up the slack and go over voltage. Imagine a 2 cell Lipo. Each cell can go to 4.2 volts so the charger applies 8.4 v to charge it. If one cell fails and only takes say 1 v, then the other cell now gets 7.4 v. When a cell goes much above 4.5 v it bursts into flames. You can get away with not balance charging, if you are lucky. However, I nearly burnt my house down doing just that!

Only ever balance charge and always use a purpose built balance charger.

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☺️Thanks for the replies Mark and Andy.

I'll definitely be staying as I am .....no wish to burn anything down but certainly not my flying machine (as my family refer to it).

Stay safe,

Guy

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