Alternative uses for your battery

Mussels

Esteemed Pedelecer
Jun 17, 2008
3,207
8
Crowborough
I'd like to power a camping fridge (42W) or charge mobile phones or even power a laptop when staying at a campsite, I have electrics but I'd like to be able to stay away from the power socket occasionally. I was thinking about getting a leisure battery but then remembered I have an expensive high tech battery sat at home that should work much better than a car battery. I can take the charger and top up the battery each day. Everything I want to use is very low power compared to an ebike so shouldn't cause any harm to the battery, I'd also need to make sure the V cutoff is the same as set in the bikes controller.
So how easy would this be, do I need to supply low voltage cutoff or is it built in to the BMS? How do I drop the voltage to a suitable level?
I can probably work it out myself but I'm sure others on here will come up with a more elegant solution and think of safety aspects.
 

z0mb13e

Esteemed Pedelecer
Jul 28, 2009
578
3
Dorset
An interesting idea and one that I might want to consider myself. I have lots of low power ancillary stuff in my camper that usually runs from the leisure battery and no electrical hook up, and after an extended stay without so much as running the engine, I found myself patching into the engine battery to get heating (gas fired, so only needs power for a fan and control circuitry), so something like this may be useful on occasion.

I would first find out about the cutout. I would expect it to be part and parcel of the BMS but I would first try discharging a battery whilst monitoring the voltage and see if/when it cuts out. What you use for a load is another matter. Something that wouldn't take too much current should the cut out not be in the BMS.

Once you have established that fact, then you can look at some sort of controller - I've been playing with the Arduino recently and as a result I would think a voltage monitor and cut out could be implemented fairly easily if needed.

To drop the voltage from 36v to 12v I would try to find an off the shelf fixed voltage regulator (though the input voltage (if 36v) is quite high and may present problems). You might need more than one if you have a lot of stuff to power, particularly things like fridges, which at 42W is going to suck 3.5A at 12v which is a tall ask of a voltage regulator. From the 12v you can easily drop that down to 5v for charging phones over USB.

Alternatively, you could use an Arduino (powered by a simple voltage divider and regulator) to create a PWM power supply using high current FETs. The added advantage there is you could easily run 6 PWM channels to create different voltages. Though they would require additional smoothing and filtering I suspect... also I'm not sure of the likely accuracy of the resultant power supplies, though it would also be possible to have the arduino monitor the output voltages and adjust them according to load...
 

z0mb13e

Esteemed Pedelecer
Jul 28, 2009
578
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Dorset
Thats a nice easy option with plenty of current. It could be easily built into a base for the battery with some cigarette lighter sockets, that way you could use off the shelf chargers for most things...
 

Trevor Holloway

Pedelecer
May 4, 2010
136
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I would be very wary of trying to power a fridge off the bike battery, surely it would last less than a day.
We find the freezer blocks (or bags of ice from the supermarket) and decent cool box worked best for a week-end.

The first time we used the car battery to power a 12v coolbox killed the battery, leisure batteries are not too expensive and easily charged / stored / transported.
 

z0mb13e

Esteemed Pedelecer
Jul 28, 2009
578
3
Dorset
I would be very wary of trying to power a fridge off the bike battery, surely it would last less than a day.
We find the freezer blocks (or bags of ice from the supermarket) and decent cool box worked best for a week-end.

The first time we used the car battery to power a 12v coolbox killed the battery, leisure batteries are not too expensive and easily charged / stored / transported.
I would expect the 42W fridge to last about 12hrs on a brand spanking new Wisper 14Ah 36v battery.
 

Mussels

Esteemed Pedelecer
Jun 17, 2008
3,207
8
Crowborough
I would be very wary of trying to power a fridge off the bike battery, surely it would last less than a day.
We find the freezer blocks (or bags of ice from the supermarket) and decent cool box worked best for a week-end.

The first time we used the car battery to power a 12v coolbox killed the battery, leisure batteries are not too expensive and easily charged / stored / transported.
I've been doing my maths and although 50w sounds like a low draw it would drain the battery in just a few hours, I'm not going to risk a £400-500 battery doing that. A 12V 110Ah PbA will only manage a day and get destroyed in the process but as it costs way less than my Wisper battery it's worth the investment.
Going down the PbA route I found this interesting article which in a nutshell says get the cheapest non sealed lead battery you can find, rapid charge it from an alternator (conveniently I have a 160A one in my car) and top up the water. Time for me to ring round and find a big 2nd hand PbA battery, I found a good one last weekend for a tenner but needed it to sell my old car.
Sterling Power Products: What is the best battery to use for an auxiliary charging system?
 

Synthman

Esteemed Pedelecer
Aug 31, 2010
417
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Oxford
I posted a similar question on a different part of the forum and didn't get a response.

I have an all-in-one computer that I would like to run off the bike battery in rare circumstances, i.e. lengthy power failure. The PC does not have a power supply built in, it uses an external one like on a laptop.

The power supply is 19.5v DC at 6.7a. I was thinking of stepping the voltage down to 12v and using a standard power inverter, but I think I read somewhere that laptops don't work on them for some reason, and I assume my PC might have the same problems.

The other plan was to do away with the power inverter and adapter completely and find a way of stepping the voltage down to 19.5v and connecting it directly to the PC, but I wouldn't attempt that because I don't want to fry the best computer I ever had!
 

z0mb13e

Esteemed Pedelecer
Jul 28, 2009
578
3
Dorset
Stepping down to 12v and then using an inverter to power your all-in-one is probably the easiest option. I can vouch for a laptop charger working on a Maplin 300W inverter.

Obviously you lose power in the step down to 12v and then again in the step up to 240v and then back down again to 19v. The best option is finding a DC charger/psu for your laptop that can accept the batteries input voltage, but they are likely few and far between. You could of course build a PSU to run directly off the ebike battery, but knowing that some laptops use signalling between the PSU and the laptop to ensure that you are using the 'correct' charger and will slow the laptop and refuse to charge the battery if not, I would be cautious about this approach.