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Discussion Starter #1
So as I posted before we went all out this Xmas and got my 3 year old son his first EV to set the trend.

We got this ; Broon T870. Amazing vehicle.

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Anyway. On boxing day I managed to crash it full speed using the remote control function with my boy on board straight into a fence post side on as I was 25m away a misjudged a turn and the distance from the post! So I got in contact with the Korean maker and to my luck they had no agents for spares for this model in the UK so the agreed to sell direct to me via Paypal. As parts where cheap (whole new bumper assy with LED lights abd grill $30! but carriage more £$40 I also ordered 2 x spare 24v battery boxes of their hot swap design for $20 each, no change I carriage.

But I needed to buy my own batteries in the UK as shipping was expensive for the weight of 4 x 12v SLA batteries needed.

The battery box design in neat and looks like this;

File 15-03-2017, 21 03 07.jpeg


So when I unbolted the original battery box it had 2 x 12v 7.2ah (20h) batteries inside connected in series to make 1 x 24v battery.

I ordered off Ebay 2 different makes of batteries the same size but 12v 9ah (20h). one is 2 x the Ultramax SLAUMXNP9-12, (£34 for two) the other two were "universal power deep cycle battery 1290 12v/ 9ah / 20hr (£32 for two).

So I have some questions for any experts here ;

A) As the new 9ah batteries are 25% more AH than the original 7.2ah ones will they last approx. 25% longer before running down?

B) when used in series in the battery box do you get 24v @ 7.2ah (or 9ah) or 24v @ 14.4ah (or 18ah) ?

C) I could have bought cheap 12v 7ah batteries for £20 for two , or expensive MK brand mobility scooter 12v 7.2ah ones for £45 for two. Would mobility scooter branded / designed ones last longer in use for time of running the car?

D) It seemed 10ah versions where taller (115mm) and 12ah versions wider (98mm). I assume you cant buy one piece 24v high AH batteries to give longer running time?

E) The charger that came with the car for the battery box is a uk 240v 0.45ah 3 pin with a 15v @ 2 amp output. Is there a better maybe faster charger to use to charge these batteries quicker without killing them? The current charger takes 8-12 hours to fully charge a 24v battery box.

In an ideal world with 3 x hot swap boxes would like to recharge one 24v battery box in about 2-3 hours max to keep the car constantly going in the summer, 1 in car, 1 on charge, 1 charged waiting to swap. Is this possible.

F) it is possible for a lot of money to buy these lithium replacement versions , any advantage apart from faster charging and more charges before they die (2000 vs 300-400).



Li7-12, 12v 7Ah Lithium Iron Phosphate, LiFePO4 Battery,
 

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A) As the new 9ah batteries are 25% more AH than the original 7.2ah ones will they last approx. 25% longer before running down?
All other things being equal, yes. However, you are quoting the 20 hour capacities; in your application you are discharging it much faster than 20 hours, so you will get less actual amp-hours depending on the battery type. Since what you actually care about is watt-hours rather than amp-hours, the voltage is also relevant- a better battery will have a voltage that sags less under load (but needs more metal in it to achieve that, so is bigger or gets you less battery and so less nominal amp-hours in the same physical size).

B) when used in series in the battery box do you get 24v @ 7.2ah (or 9ah) or 24v @ 14.4ah (or 18ah) ?
If you put them in series, the amp hours stay the same and the voltage doubles.
If you put them in parallel, the voltage stays the same and the amp-hours double.

C) I could have bought cheap 12v 7ah batteries for £20 for two , or expensive MK brand mobility scooter 12v 7.2ah ones for £45 for two. Would mobility scooter branded / designed ones last longer in use for time of running the car?
You need batteries matched to the job. Batteries built for higher current will get less notional capacity in a given size box, but deliver more of the available energy to the terminals rather than heating the battery internally.

So if you buy a battery that's designed for a lower current than you are taking, the battery will get hot and you will get less energy out of it than it says on the label. Convesely, if you buy one designed for higher current than you need, then it will work nicely but will have a lower capacity than an ideal battery of the same dimensions.

D) It seemed 10ah versions where taller (115mm) and 12ah versions wider (98mm). I assume you cant buy one piece 24v high AH batteries to give longer running time?
There's not much point in having one-piece 24V batteries, since they would be in effect two 12V batteries stuck together. You theoretically save a tiny amount of the casing between them, but the overall thing is heavier so the outer casing has to be stronger/thicker - taking you back to where you started. In general, as you go to bigger capacities you go down in voltage to keep the individual units of manageable weight - so going significantly bigger you'd probably be using four 6V batteries, and bigger still individual cells.

E) The charger that came with the car for the battery box is a uk 240v 0.45ah 3 pin with a 15v @ 2 amp output. Is there a better maybe faster charger to use to charge these batteries quicker without killing them? The current charger takes 8-12 hours to fully charge a 24v battery box.
The datasheet for your battery says it can take a maximum of 3.6A at a maximum of 14.7V. So you might expect an ideal charger to charge it in 3-4 hours to nearly full.

F) it is possible for a lot of money to buy these lithium replacement versions , any advantage apart from faster charging and more charges before they die (2000 vs 300-400).
Unfortunately that one has a rather crappy translated-from-the-chinese datasheet, but as well as charging twice as fast you'd expect it to be lighter - which will give extra run time through less power needed to move its own weight (though probably it's insignificant in relation to the weight of your son?).
 

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Discussion Starter #3
Thank you very much. Great help. Another question on charging. As the battery box makes 2 x 12v batteries into 1 x 24v unit, which is then bolted down and used as one 24v unit for power and charging with a power/off/charge rocker Switch. Is this why the 24v unit takes so long to charge off the supplied 15v 2amp charger. It is kind of trickle charging. Could u use say a 24v 3amp output charger to charge a battery box unit in 3-4 hours?
 

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Thank you very much. Great help. Another question on charging. As the battery box makes 2 x 12v batteries into 1 x 24v unit, which is then bolted down and used as one 24v unit for power and charging with a power/off/charge rocker Switch. Is this why the 24v unit takes so long to charge off the supplied 15v 2amp charger. It is kind of trickle charging. Could u use say a 24v 3amp output charger to charge a battery box unit in 3-4 hours?
Certainly I'd expect to treat it as a 24V unit and charge it at around 28V 3A. If your existing charger is rated 15V and you're connecting it across the full 24V then I'm surprised it works at all. Maybe that rocker switch is re-wiring the batteries into a parallel arrangement for charging - in which case the 2A from the charger is only 1A into each battery and certainly much slower than the batteries can take.
 

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Discussion Starter #5
Certainly I'd expect to treat it as a 24V unit and charge it at around 28V 3A. If your existing charger is rated 15V and you're connecting it across the full 24V then I'm surprised it works at all. Maybe that rocker switch is re-wiring the batteries into a parallel arrangement for charging - in which case the 2A from the charger is only 1A into each battery and certainly much slower than the batteries can take.
I suspect it does do this as the charger is small and has a led on t that starts red then moves to a reddish amber then green when the battery s fully charged. But it does take at least 8 hours+. It took 3 hours last night to fully charge the new 9ah batteries in their battery box when they arrived from the ebay supplier.

At the weekend I will connect a multimeter to the single quick output connector on the base of the box and play with the rocker switch and measure the voltages. If it does give the readings 24v/0v/12v then it works as you suspect, and I can look into buying a 24v-28v charger than gives 3 amps to spped up the process and charge it as one unit in the "power" rocker position, assuming the power connector in the top is connected to both batteries when the switch is in the power position.

Thanks Again.

M
 

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Discussion Starter #6
Certainly I'd expect to treat it as a 24V unit and charge it at around 28V 3A. If your existing charger is rated 15V and you're connecting it across the full 24V then I'm surprised it works at all. Maybe that rocker switch is re-wiring the batteries into a parallel arrangement for charging - in which case the 2A from the charger is only 1A into each battery and certainly much slower than the batteries can take.
Your thoughts on this please Arg , could I mod this 24v 3 amp mobility scooter charger using the inline XLR female plug soldered to a 2 connection centre pin input of the battery box? I guess I could pick up one of these round centre pin connectors of the right size from Maplins.

PRIDE REVO 24v - 3a SCOOTER BATTERY CHARGER | eBay

XLR SOCKET 3 PIN, MOBILITY SCOOTER POWER CONNECTOR 24V BATTERY 10A HIGH POWER | eBay

Although i'm not sure about how XLR connections work as they seem to have 1 positive and 2 negative connections? Could I get away with just using 1 of the negatives and the positive to adapt to plug into the car 24v battery box ?

My daughter has a 24v electric kids moped with this 3 pin charging system, this is my next project to look at the batteries under the seat and upgrade those as they are not hot swappable and fixed in place out of reach.

M
 

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Well, it should work - but there's a lot of variables here, and getting things wrong has a fair chance of either killing your valuable batteries or setting things on fire.

Your plan assumes you can rewire the packs to present the cells in series to the connector which we believe puts them in parallel for charging. I might be tempted to wire an XLR connector directly to the pack and leave the barrel unchanged. Having an in-line fuse inside the pack might be a good idea (if there isn't one already).

I'd be reluctant to trust a random Ebay purchase without carefully measuring what it actually does (current/voltage)- but then I've got suitable meters lying around and you maybe haven't.

No problem with using only two pins out of the XLR if the cable wires all three - the current is the same in both wires, so if a single pin is sufficient for one it's implicitly sufficient for the other. I think some of these devices use the fact that two pins are connected together to disable driving while it's plugged in.

Beware that barrel connectors come in two similar sizes (2.3mm and 2.5mm), where a 2.3 socket will accept a 2.5 plug but make poor contact. A quick browse through some catalogues shows that most of the 2.5mm ones seem to be rated 5A so would be adequate, while many of the 2.3mm are only rated 1A.
 
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