Charging batteries question.

[OxygenJames]

Greetings.

This is actually about an electric scooter I have - but I am assuming the technology is the same as on a bike - well I am guessing there are plenty of people here who will have an idea how to help me (I also do own and use a very nice Haibike electric bike which I claim as my passport for asking this question!).

So.

My scooter had two 12V lead-acid batteries. Which had one 24V charger that you plug into it to charge it up. So far so simple yes? OK. So during one of their super-deal days on ebay there came the chance to get two replacement brand new 12V lithium batteries for £90 (usually they cost around £140 for two) - so I went ahead and bought them.

They arrived and they look perfect they're the right size and weight considerably less than the lead acid (and I am thinking will last longer and I seem to have to replace the lead-acid ones every year or so). Anyhow. So here's the thing. They came supplied with 12V chargers - somewhat strangely they sent me two - one for each battery?

But. Seeing as I will wire these up the same way they were wired up before - in series - that means I will have - effectively - one 24V battery.

Various questions come to mind:

1) Should they have sent me one 24V charger?
2) I assuming I can not use the old charger that was for the lead-acid batteries. Correct?
3) Will one of the chargers work to charge both batteries when wired up together in series?

Otherwise it would seem I have to disconnect them and wire up the two chargers separately everytime I want to charge them (or charge them one at a time but having first disconnected them from eachother).

Any thoughts?

Cheers.

James.

 

[OxygenJames]

Or is the simple answer that I need to buy a 24V lithium-battery charger?

Costing it would appear on ebay about £8.

 

[Nealh]

Do the 12 v Li batteries each have a BMS ?

You can't risk charging li in series as any cell will exceed the top voltage, you could though charge in parallel as 12v if no BMS is used. For li the batteries need to be singular for charging and not connected up in series as acting as one.
Chemistry and charging regime does differ between Li & SLA.

 

[OxygenJames]

Hmmmm. OK. No idea if the batteries have a BMS.

Here's what they look like:

(are you sure I can't just wire them in series and buy a 24V charger?)

 

[sjpt]

Do you usually charge them on or off the scooter? If off, just use your two chargers. If on it will be more complicated as Nealh says.
Also, the max discharge current of 10A may be a little low depending on the controller and motor; it doesn't say what the continuous discharge current is.

 

[OxygenJames]

In fact I am starting to wonder if it's even OK to wire these up in series.... If you're saying you can't charge them in series with a 24V charger because any one cell may get too much voltage - then doesn't that imply that to even have them run in series would cause the same thing?

Thoughts anybody?

 

[OxygenJames]

Do you usually charge them on or off the scooter? If off, just use your two chargers. If on it will be more complicated as Nealh says.
Also, the max discharge current of 10A may be a little low depending on the controller and motor; it doesn't say what the continuous discharge current is.

There is one charging point on the scooter. The batteries are positioned within the structure of the scooter and it's a bit of a pain to get them out. To charge them separately would require some amount of new wiring or taking the scooter apart everytime I wanted to charge them.

I am still wondering if this means I should not even be wiring them up together in series......

 

[Nealh]

There not true lithium but lifepo4 /li-iron phosphate /LFP they will last for ever, Cycling them correctly and they will outlast lion/lipo by 4X. They have a very narrow charge/ discharge voltage of 3.2v - 3.65v.
If they don't have a BMS, you can charge in series using a lifepo4 24v charger but keep an eye on any one of the 8 cells from wandering they need to be kept close in voltage. You need a specific 24v lifepo4/LFP charger and not a 24v lion/lipo charger

Because of the narrow voltage range the cells ideally need to be kept within 0.05v. If cells do wander one will have to charge each battery separately.

In series they will discharge evenly, it's the charging process that may cause issues .
One should check their discharge voltage before charging.

 

[Nealh]

They will likely have a BMS though as they have 4 cells each charging in series and will be monitored, it isn't a good idea to charge in series. If one battery charges before the other then a bms will shut off charging.

One can join and discharge in parallel and in series they may be ok, but again the BMS will shut down if one cell is less and reaches lvc first.
They are only 10a rated so in series still 10a.

 

[OxygenJames]

They will likely have a BMS though as they have 4 cells each charging in series and will be monitored, it isn't a good idea to charge in series. If one battery charges before the other then a bms will shut off charging.

One can join and discharge in parallel and in series they may be ok, but again the BMS will shut down if one cell is less and reaches lvc first.
They are only 10a rated so in series still 10a.

OK thanks.

It seems what you're saying is:

1) You can wire them up in series and they will work fine as a unit.
2) Charging them in series could be an issue so probably best to charge them individually.

If I was to take the chance to charge them in series - would this charger be the right one?

Thanks again for your time and thoughts.

James.

 

[Nealh]

Yes should do the job like a lot of lipo chargers it has multifunction charging so all one needs to do is select the correct battery chemistry in the settings and charge current.
The chemistry is a lot more stable then lion or lipo.

In series no reason why they won't work, however as mentioned one wayward cell will stop any discharge. Series connections with inbuilt bms therefore may give issues.
On can try the series route for charging but keep an eye on cell voltages until you are happy they are ok. You may find that if one battery reaches full charge before the other then charging will cease and one battery may not be fully charged.
If charging singularly always make sure before connecting in series again that both battery voltages are equal.

SLA don't use bms so series connection is easier as is charging, the lifepo4 will give more life and greater power over the discharge cycle.

 

[OxygenJames]

Yes should do the job like a lot of lipo chargers it has multifunction charging so all one needs to do is select the correct battery chemistry in the settings and charge current.
The chemistry is a lot more stable then lion or lipo.

In series no reason why they won't work, however as mentioned one wayward cell will stop any discharge. Series connections with inbuilt bms therefore may give issues.
On can try the series route for charging but keep an eye on cell voltages until you are happy they are ok. You may find that if one battery reaches full charge before the other then charging will cease and one battery may not be fully charged.
If charging singularly always make sure before connecting in series again that both battery voltages are equal.

SLA don't use bms so series connection is easier as is charging, the lifepo4 will give more life and greater power over the discharge cycle.

OK great thanks.

For now what I've done is wire up a cut-off switch between the two batteries so that when I charge them (individually) they are not linked in any way. I've also wired some terminals outside the scooter so I don't have to take the thing apart everytime I charge it.

It's very Heath Robinson but it works fine and this way I can be sure both batteries will be charged properly (it's also cheaper than buying another charger which may not work anyway!).

So thanks for your help - I was about to buy a regular Lithium 24V charger which by the sounds of things would have just screwed everything - you've saved me from that FU.

All the best,

James.

 

[Nealh]

Yes Lion and lifepo4 very different voltage charging.