Battery wiring

[charlesv842]

I have a Razor E300 scooter. It's a 24 volt system-two 12 volt SLAs with a wall charger. I'm pretty sure I should be able to add another battery in parallel with one of the existing batteries, right? It won't have any problem charging the extra one will it?

 

[dnmun]

yes. the capacity of the two 12V in series on one side has to be close to the value of the capacity of the two 12V batteries in the parallel string. ideally you want the internal resistance to charging to be identical on each side.

 

[wesnewell]

As long as you keep the capacity the same between the 2 in parallel with the single battery, you'll be ok, but don't parallel say another 5ah battery to 2 5ah batteries you have in series. If you do, you'll ruin the single battery by over discharging it. If you're trying to get more range, your best bet is to replace the lead batteries with lithium.

 

[amberwolf]

EDIT: link below to clarification
http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=70171&p=1059081#p1059081

EDIT: below is *partly* true, but based on a misunderstanding of what the OP wanted to do. The below only works if adding a larger capacity battery of the same voltage as the *entire series string* of smaller capacity batteries, paralleled to the *entire series string*.

It will *not* work to parallel to only *part* of the series string of smaller batteries.

:? I don't understand how paralleling a larger battery with a smaller one is going to cause overdischarge of the smaller one? It's never happened when I've done that, either with SLA + SLA or with NMC + RC LiPo.


Everything in parallel is going to be the same voltage all the time (assuming your wiring is up to the current flows), so as long as all are either the same chemistry or have the same (maybe even just similar) discharge curves and HVC/LVC points, I don't see how a problem would arise.

It might not even matter about the discharge curve, if either pack on it's own can supply the current to run the bike, becuase then whichever one is higher in charge state (presumably lower in Ri) at the time will do that, while the other will supply little of the current untl the first begins to run down.


If you used two packs that have different HVC and LVC points, and simply parallel them while they are at a point on their curve that makes them the same voltage, then *that* could cause a problem once you discharge the pair far enough to pull one of them past it's LV point while the other is still able to continue, assuming neither is protected by a BMS. (and same for charging them)


If you have to use two packs that are very different, but either could run the bike, you'd have to use a switch to toggle between them...but that isnt' teh best way to do things. Directly paralleling two packs that either are the same or have the same HVC/LVC points will make the load on each pack easier and give you better performance (and with SLA, better range).

 

[wesnewell]

Take 2 7ah 12V batteries in series and they should discharge fairly evenly down to 21V from a charged voltage of ~26-27V. Recommended minimum discharge of a 12V SLA battery is 10.5V. Now parallel one of those 7ah batteries with another for 14ah and put in series with the single 7ah battery for the same charged voltage. When the voltage has dropped to 21V, which should be the lvc for a 24V system, the single battery will be below 10.5V and the paralleled batteries will still be well above 10.5V. It's that simple. The same thing would happen with 2 batteries in series if one has a higher capacity than the other. That's why you don't series a higher ah battery with a lower ah one. And I can guarantee it's happened to anyone that done it, even if you didn't notice it. You just may not have run the pack down low enough. It's fine as long as you don't use more capacity from the pack than the lower ah rated one is rated for. You can draw 7ah from a pack with a 14ah battery in series with a 7ah battery without over discharging. After that you are over discharging the smaller battery.

 

[charlesv842]

I don't quite follow all the terminology there, but it seems like it would cause a problem having different capacities linked in series. They would both charge and discharge at different rates? I'm guessing if both capacities were equal, then it would all work. Does that sound right? I'm gonna need to add an extra battery to both originals, and they will have to be the same capacity, correct?

 

[DAND214]

charlesv842 wrote:
they will have to be the same capacity, correct?

They could be the same, larger or smaller. As long as the total capacity of each parallel group are the same AH.

Dan

 

[dnmun]

like AW said, they will both charge up, and the side with more capacity will take more of the charge because the side with lower capacity will build up in voltage faster forcing more current into the side with more capacity.

but you will get it all back, as the side with lower cpacity drops in voltage faster then the side with more capacity will produce more current to the load.

if each side is balanced in resistance and capacity then they will fill up together at the same rate. if one side has higher resistance to charging then it will fill up slower because the voltage on it will be higher than the other side with lower resistance so more current goes to the lower resistance side.

 

[dogman dan]

In case it wasn't clear from the above....

You have two 12v batteries connected in series, to make it a 24v battery.

To add more, you have to have two more 12v batteries to connect in series, then you can parallel connect the one 24v battery with the other. The new 24v battery does not have to be the same size cells as the original 24v set.

Adding just one more 12v battery won't work, unless you want to make a 36v battery, by connecting it in series with the 24v set.

 

[amberwolf]

Take 2 7ah 12V batteries in series and they should discharge fairly evenly down to 21V from a charged voltage of ~26-27V. Recommended minimum discharge of a 12V SLA battery is 10.5V. Now parallel one of those 7ah batteries with another for 14ah and put in series with the single 7ah battery for the same charged voltage.

Ah, ok, that's what I missed.

I somehow misread what the OP wanted to do. Sorry about that!


What I originally saw was that the OP wanted to parallel a larger battery of the same voltage as the entire series string of smaller batteries with the entire string of them. *That* I have done wihtout issue.

What the OP actually wants to do is parallel one larger battery of half the voltage of the entire series string of smaller batteries, with just one of the batteries in the string. *That*, as you show, won't work, and I haven't done that for that reason.

I've added a pointer to this post from my original one so I don't confuse the issue (as much).

 

[Ultraman]

You might want to get rid of the lead batteries altogether. I've taken apart 4 of these 24v Razor scooters and replaced the lead batteries with lithium cells. Gives a much more robust ride, reduces weight, and speeds things up if you want to go faster. You'd need a new charger, but you could buy a low amp 36V Meanwell from ebay for this. The conversion project might come out to cost as much than the scooter itself, but if you use it often it would be worth it. I had a bunch of old 18650 cells lying around so my cost remained low. But even if you buy new lithium batteries, you could get away with 10s4p (40 cells) which would dramatically increase your top speed and your range at the same time (2.5ah cells x 4 = 10ah). The motors and controllers in these scooters are capable of 40+volts no problems.