Lipo booster

[awol]

To get used to handling/charging lipo batteries I decided to buy a singe 2s 7.4v 6aH lipo and join it in series with my 36v li-ion bottle battery which also gives about 6aH before flat. It's all wired up in series and appears to work riding up and down my road with a great speed/power improvement. Watt meter shows 48v.
I have a lipo alarm on it and a 40a fuse on it's positive between it and the controller.
Before I set off on a lengthy ride I just wanted to check if others have done this and if it's safe to do so.

 

[Deleted member 4366]

The controller should switch off at about 31v to protect the battery. The battery should have LVC in its BMS, but in my experience, they don't always work. The controller's LVC won't shut off until too late because it can only see the total voltage of the two batteries. therefore, for insurance, you should have a real-time voltmeter on your main battery until you can prove that its LVC is working.

 

[awol]

As an experiment, I just connected the watt meter between my main battery and the lipo and I get the volts reading of just the main battery. So could monitor it that way maybe?
I was more concerned about mixing the li-ion with the lipo. Am I ok to asume that as long as I know before either needs charging then the mix is ok? as it takes aprox 4 to 4.5aH to get to work and back normally.

 

[Deleted member 4366]

I can't see any reason for concern. Try it, and tell us how it goes.

 

[KirstinS]

Please do report back - I have a booster battery but use laptop cells which do suffer from sag when being worked hard

Most interested to know how lipo works out

I always worries about it killing my main battery as that is also 18650 lion cells. Higher C rating than the laptop ones but nothing like lipo

For some reason I always assumed that you want the worse sag/performing battery on the booster so that it doesn't take down your main battery

Though now I write it down I have no idea why I thought that . I probably read (or misread!) something on Endless Sphere !

 

[awol]

Two days and 40 miles so far so good, it works great with a good increase in power and speed, and no noticable voltage sag, however I do not know if there is any underlying damage being done to my main li-ion battery so will do some discharge tests on that in the future.

As for handling/charging, I decided I did not like handling the battery directly so have encased it for safety with a fuse with an xt60 socket to plug into which would be essential if I were to go down the 12s route in the future.

Day 1 I got home and my lipo checker showing only 6% of it's 6ah capacity remaining 3.57 3.63v (and still getting full power just before the check) but I only normally use about 4.5aH max.
Day 2 my wattmeter says my 20 mile trip used aprox 4aH which I believe should leave aprox 35% of the lipo's 6aH but my lipo checker says only 17% 3.71 3.72v remaining.

So lipo does seem to use more aH and discharge quicker, any idea's why would that be? as it would affect my 6s aH battery capacity choice in the future.

 

[Deleted member 4366]

4ah for 20 miles sounds about right. When you say "quicker", quicker than what?

 

[awol]

4ah for 20 miles sounds about right. When you say "quicker", quicker than what?

It seemed to loose capacity quicker than my usual li-ion.
I mean my li-ion normally used 4.5ah but the lipo used nearly all 6ah 'ish.
Maybe it's because I am up to 44v or they do just discharge faster than li-ion therefore I would need a higher aH capacity 12s in the future to do the same current 4.5aH li-ion trip.

 

[Deleted member 4366]

I think on day one, you were enjoying the extra power. Day two was 4ah, which is what you normally use?

Thanks for sharing.

 

[awol]

Day 1 definitely enjoying new full throttle !! Got to work nearly 10 mins quicker !

 

[awol]

It seems I got my calculations wrong.
My 6aH booster lipo only gave about 17 miles before alarming just before I got home so the 6aH capacity is no good BUT more importantly does not match the "supposed 6.5aH" capacity of my bottle battery which my wattmeter suggested it gave before cutting out, which confused me.
So plan-B was I went ahead and added a second 2s lipo 6aH to connect in parallel with it. Encased together they now give me a booster battery of 7.4v12aH which after tests does about 35miles which is good power wise but not good cost wise and also means I am carrying more weight.
The good thing is I don't really need to monitor it as I know it easily does the whole trip and they can both get charged at the same time.

 

[Deleted member 4366]

Now you need to monitor your other battery. Does it cut off at 31V? Personally, I wouldn't let it go below 34v.

 

[awol]

I will add a voltmeter to the bottle battery and keep an eye on it, thanks.

 

[awol]

Everything still running great and smooth at 44v. I've been monitoring my main 36v bottle battery for a while and it seems unchanged from the start, volts drop to 32.5v under full load at about 20miles up the last incline then recovers to about 37.5v so I've noticed voltage sag of about 5v on it under load.
Just curious how many volts would the equivalent 10s lipo or even 12s sag under load?

 

[Deleted member 4366]

It depends on the C rating. Some hardly sag at all. When they're old, they sag more though.