Jump to content
steelo

Lipo questions

Recommended Posts

I apologize if this has been asked before, but I was looking at some 2000mah 2s lipo's that seem to be decently priced. Is there any benefit in run time over, say a 2000mah nimh since the capacity is the 'same'? I know there are other benefits, such as longevity, durability and no 'memory' effect over nicads and nimh's. Does the vehicle slow down as the lipo discharges or does the battery alarm/cutoff stop it before that?

Also, I know I will need a low voltage alarm since I'm using Tamiya ESC's in 2 out of 3 of my cars...I noticed a lot of the alarms need to be set by the user. What would be the proper voltage cut off to prevent damage to the battery? The Arrma ESC in my Neo Scorcher is supposed to cut off at 3.2v/cell, is that a good setting for an alarm?

Thanks!

Share this post


Link to post
Share on other sites

2000mah is the same no matter what the battery but LiPo will run a bit longer and die fast where NIMh will fade away after half distance. Alarms are about £2, most seem to be made for up to 4s with no setting up needed.

Fitting in the chassis is the main issue with liop

Share this post


Link to post
Share on other sites

The discharge curve doesn't drop in voltage so much as the charge depletes (LiPo vs NiMh/NiCd). Coupled with the fact that the internal resistance of LiPos is a lot less, you notice much less slowing of the car over a run. Because of the lower IR, less heat is generated in the battery and so more of the capacity in the pack can be delivered to the motor so a theoretical runtime increase could be gained if the gearing is reduced to compensate for the higher voltage and lower IR. In reality, most people will leave the gearing so you probably won't see a massive run time difference.

The voltage you set on the alarm is best tailored to the amount of power you are drawing. If you are running a silver can, for example, and drawing a low current as a result, set the alarm higher. Maybe 3.4V/cell (people have a lot of different views about cutoff voltage values). The reason is that if you are drawing a low current, the resting voltage of the battery is closer to the voltage under load. You don't want the resting voltage to drop too low, that's the critical point. On a powerful motor, you will need to reduce the cut off voltage or the alarm will go off every time the throttle is opened, but the resting voltage could still be at a safe level.

Share this post


Link to post
Share on other sites

Thanks guys!

One more question, Do brushless motors (with roughly the same number of turns) draw less current than their brushed counterpart since they are more efficient?

I have a Sand Scorcher with a torque tuned motor (27T) and a M-05 with a 13.5T brushless. I would imagine the brushless would still draw more since it has half the number of turns...meaning I would set the alarm at a lower voltage.

Share this post


Link to post
Share on other sites

Yes the 13.5 will draw more. All other things being equal. Even in the much lighter car with front wheel drive I think it still will.

Share this post


Link to post
Share on other sites

Brushless motors draw a lot LESS current for a given amount of power because they are much more efficient than their brushed counterparts.

As for what number of turns matches power for brushed & brushless it's hard to say. Brushless performance depends greatly on the ESC and it's dynamic timing controll. Though I'm pretty sure a 13.5T brushless is most comparable to 19T brushed motors.

Share this post


Link to post
Share on other sites

For a given amount of power I agree, BL will draw less. But a torque tuned 27T will be making way less power than a 13.5 BL under all conditions.

Share this post


Link to post
Share on other sites

I apologize if this has been asked before, but I was looking at some 2000mah 2s lipo's that seem to be decently priced. Is there any benefit in run time over, say a 2000mah nimh since the capacity is the 'same'? I know there are other benefits, such as longevity, durability and no 'memory' effect over nicads and nimh's. Does the vehicle slow down as the lipo discharges or does the battery alarm/cutoff stop it before that?

Also, I know I will need a low voltage alarm since I'm using Tamiya ESC's in 2 out of 3 of my cars...I noticed a lot of the alarms need to be set by the user. What would be the proper voltage cut off to prevent damage to the battery? The Arrma ESC in my Neo Scorcher is supposed to cut off at 3.2v/cell, is that a good setting for an alarm?

Thanks!

I am also about to use a Tamiya EXC with my 20c Core Lipos (which are an exact fit for all old-school nimhs). I am interested in knowing which alarms you think are best. Any advice?

Share this post


Link to post
Share on other sites

Nimh will start to drop off quickly from the moment you roll.. Lipo is just stay there until flat. lipo is like nimh with Viagra. but never never ever go flat on lipo, it will damages the battery by puffing and catch on fire. so you set the cut off voltage 3.2v per cell (2s lipo) so u set 6.4v per battery. on ESC or lipo warning.

I hope you have lipo capable charger with balancer to charge the batteries.

Share this post


Link to post
Share on other sites

Okay, a little confused...as far as for storage, I've heard to keep a 2s lipo at 3.8v/cell. Isn't a 2s only 7.4volts? wouldn't that equal out to be 2cells x 3.8v=7.6v????

Share this post


Link to post
Share on other sites

A fully charged 2S lipo will be 8.40V - in the same way an old 7.2 Nimh will actually be upward of 9V when charged. Some charges actually have a storage mode. If I don't use my lipos for any length of time I store them at around 7.8 - 8.0 V. So long as the're not fully charged or at the point of full discharge, you should be ok. If you store them for months at a time it is worth checking and topping up if necessary.

I set the cut off on mine for 3.4V and usually have had enough or break something before it kicks in (although mine are 5300 and 4500 mah respectively).

Share this post


Link to post
Share on other sites

Nimh will start to drop off quickly from the moment you roll.. Lipo is just stay there until flat. lipo is like nimh with Viagra. but never never ever go flat on lipo, it will damages the battery by puffing and catch on fire. so you set the cut off voltage 3.2v per cell (2s lipo) so u set 6.4v per battery. on ESC or lipo warning.

I hope you have lipo capable charger with balancer to charge the batteries.

Really it's over charging that will cause batteries to puff and sometimes catch on fire, a slow discharge will damage the battery but won't make it puff or catch on fire. You can even bring flat lipos back from the dead if you know how.

Share this post


Link to post
Share on other sites

Agreed charging is the problem. I've had a couple of packs go under 3.0V/cell and they are still fine with no swelling, plus I managed to resurrect them.

Share this post


Link to post
Share on other sites

I know you should always monitor the charging, but doesn't a smart charger prevent overcharging?

Share this post


Link to post
Share on other sites

No, only diligence on behalf of the operator.

Some 'smart' chargers have been known to mistake a fully charged 2 cell for a flat 3 cell, for example. If that happens, you have a guaranteed failure on your hands. I would only ever trust a charger when I have told it the cell count explicitly, you don't want the charger making a guess at it. This seems to be the number one cause of charging fires. It's minimised by using a balance lead on every charge, which should mean the charger can't misinterpret the cell count or allow any cell to exceed the maximum permissible voltage.

Share this post


Link to post
Share on other sites

My charger makes me confirm the cell number manually each time and has a max time cut out, max capacity cut out and auto monitors for what charge rate it charges at, max 6a but its quite often less. I balance charge every time.

  • Like 1

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...