Upgraded batts in scooter - should I get a new charger too?

[badnewswade]

Hi all- Posting to say that I've finally solved the "rubbish range problem" on my leccie scooter - it now does 30mpc easily, at a push it'll prolly do 34-35.

First I replaced the 20ah batteries with a set of 28ah ones, then squeezed the original 20ahs into the luggage compartment on top of the 28ahs, so now I have a whopping 48ah, 48v battery.

Here's the new batteries, they sit under the seat (which I had to mangle a bit so it wouldn't crush down on them).





The question is, should I buy a more powerful charger, or even charge them seperately? (would be a real hassle to charge seperately though) I currently have a (guesstimate) 4ah charger and it seems to be taking a bit of a long time to charge up after even short runs, I just did about 10 miles today, put it on at 3.45pm and ended charging after four hours and four minutes (7.49pm - just now in fact!)

Come to think about it that might not be too bad after all, but I'm still kind of worried that what with the batts being different amp-hour ratings I might be damaging them by charging them together. What do people think? I'm considering getting yet moar batts for even moar range (maybe some 10-14ah ones) so it's quite important.

Also is it possible to damage lead-acid batts by charging them too slowly? I saw a youtube vid about a guy in New Zealand who'd built his own electric car and he mentioned that he'd damaged the batteries by charging them too slowly!?!?!?

AND... shouldn't be any problems with hydrogen venting should there? I tend to open the seat a couple of times when it's charging anyway, but they're only 20ah, and I have 15ah batts in my other, small leccie bike and they're in a much more enclosed space.

[GregsGarage]

Just a quick thought on your setup. Mismatched parallel batteries is a bad idea. Don't make it worse by adding 10ah ones as well. I would replace the 20 ah with more of the 28ah before upgrading the charger.

I am away for a week, so no more time to chat just now.

Greg

[badnewswade]

Any ideahow I could fix that? I really can't afford another 28ah pack and don't want the 20s to go to waste either. As for the idea of extending further with 10 or maybe 14ah batts, they'd be only for use on long runs and could be charged seperately.

What if I charged the 20ah ones seperately?

[gwing]

Hi Wade,

I think you might have problems on both the charging and discharging fronts.
When charging the batteries in parallel I think it unlikely you can ever achieve a satisfactory state of charge on both sets of batteries. Either the small ones will be overcharged or the bigger ones will be incompletely charged - charging each bank of batteries separately would avoid this problem but then you need a means of isolating the two battery banks.

The second problem is that when you drive the scoot the smaller batteries will be driven to a lower state of charge than the large ones and hence will wear out much quicker - and if you continue riding to a point where even the larger batteries are low this really will have done the small ones no good at all. Ideally you would ride the scoot on both sets of batteries in parallel until, and only until, the small batteries have reached your maximum dsired discharge level and then switch to just the larger batteries to squeeze a few more miles. Problem is I think that it will be difficult to know when those small batteries are getting empty.

The KISS approch might be to isolate the battery banks (which you need for charging anyway) and just ride first on one set of batts until they run low and then switch to the other set. That isn't quite as kind on the batteries as sharing the load between them but simpler to understand and implement I think.

[GregsGarage]

I don't think you should have a problem charging both banks in parallel at once with your existing charger. As the batteries reach a full charge their internal resistance to charging increases which is why you see the voltage at the charger getting higher as the batteries get full. If one bank is getting closer to being fully charged its increased resistance will force more of the current from your charger into the other bank, until they both reach a full charge and the charger switches off. Because they are parallel to each other the charging voltages will be equal across both banks.

Your main problem with this setup is what happens during discharge, particularly high current draws during acceleration or climbing a hill. The internal resistance of the batteries, along with the resistances in the wire and connectors, will determine current draw from each bank. If your parallel batteries are all the same, then internal resistance will be the same and current draw will be the same from each bank, and they will stay at the same level of charge. However having different batteries on each bank will mean you probably have different internal resistances so the current draw from each bank will be different. If the internal resistance of your 28ah batteries is a bit lower than the 20ah batteries you may find they discharge such that a 48 amp discharge is taking 28 amps from the larger pack and 20 amps from the smaller pack, in which case they will be discharging at the same rate in percentage terms (ie. both will reach 50% discharge at the same time). However, if the 20ah batteries have a lower internal resistance than the 28 ah batteries the situation on a high discharge will be reversed. You will pull more current from the smaller battery, lowering its state of charge more than the larger pack. Once the load is removed the 2 packs will be at different voltages, the pack with the higher voltages will discharge into the other pack through the parallel connection. This presents 2 problems. First is that high currents can generate heat in the wires, any excessive heat is a fire risk. And second is that you lose energy by charging one bank from the other, and so reduce range.

So your original questions were, should you buy a bigger charger and should you increase the pack size some more with 10ah batteries?

Your charger should be fine, just take longer to charge because of the larger pack capacity. I wouldn't get a larger charger until after you have got a pack with all the same capacity batteries.

I wouldn't add 10 ah batteries to this setup as well, because of the reasons stated above. Run with what you have at the moment, and when you can afford to, replace the 20ah with more of your 28ah batteries (preferably the same make). I wouldn't spend any more money on anything else except maybe 2 amp meters, one for each bank. It would be useful to know what is happening to each bank under various conditions, ideally the larger bank is doing a proportionally larger amount of the work.

[badnewswade]

Yeah, that sounds reasonable. It seems OK on hills and stuff (its only got an 800w motor so it can't pull that many amps!), and the difference between packs doesn't seem to be too much - although it did lose power after one hill I think that was to do with the motor getting hot (it was a very hot day). I check the batts and suchlike whenever anything odd happens and it's been fine, done about three or four long trips and taken its fair share of hills too.

Say, wouldn't it be good to do seperate charges after deep-cycling? I designed the extra pack so I could disconnect it fairly easily so it's not too much trouble.

(not that I deep-cycle much, most I take it is about 30 miles, which I guesstimate is around 80%- pack voltage down to around 48-49v, performance starts to fall off almost imperceptibly) Also is it better to use a smaller charger? I have just got a 2.6 amp one off the internet as a spare and to do those seperate charges, and I want my money's worth!

[GregsGarage]

Say, wouldn't it be good to do seperate charges after deep-cycling? I designed the extra pack so I could disconnect it fairly easily so it's not too much trouble... ... Also is it better to use a smaller charger? I have just got a 2.6 amp one off the internet as a spare and to do those seperate charges, and I want my money's worth!

I don't think it will matter too much, more important is that you recharge the batteries asap. Batteries don't like being left discharged. If you charge separately, be sure both banks are fully recharged before you reconnect them. The only time I use a slow charge on a battery is if it has been left discharged for a long time, then the slow charge gives a better chance of recovering the battery.