What size cable for connecting 12v batteries?

[woodman-46]

I want to connect four 12v 15ah agm batteries in series to create a 48v power source. Can someone let me know what thickness or specification cable I should use for this. Thanks

 

[JohnW2]

I want to connect four 12v 15ah agm batteries in series to create a 48v power source. Can someone let me know what thickness or specification cable I should use for this. Thanks

That depends upon what current you want to be able to draw from that 48V power source. What is the answer?

Kind Regards, John

 

[woodman-46]

Thank you for getting back to me John. I am not sure exactly what current but hopefully I can run some figures by you and you might have an idea?
The batteries will be used to power a 48v 800w brushless motor (hub motor as used on electric bikes picture attached.) I guess sometimes the throttle will be pulled back fairly hard but most of the time it would be for consistent speed. I had intended on putting a fuse between each battery unless it is deemed unnecessary but certainly a fuse somewhere in the "loop."
I hope that gives some info for any further advice.
Cheers James

 

[JohnW2]

I want to connect four 12v 15ah agm batteries in series to create a 48v power source. ..... The batteries will be used to power a 48v 800w brushless motor (hub motor as used on electric bikes picture attached.) I guess sometimes the throttle will be pulled back fairly hard but most of the time it would be for consistent speed.

Do you realise that, if it were running at it's full power (800W), you would get less than an hour out of 15Ah batteries, even when new/healthy and fully charged (although obviously longer at lower power)?

Kind Regards, John

 

[woodman-46]

Yes I didn't think it would be a very long time but enough for my young son to have a bit of fun in a paddock etc, the most ah possible for a small 12v agm battery is 15ah by the looks of it. I had looked at lithium and have already posted a topic about that but expense and concern about safety of lithium ion veered me towards agm batteries (I know how heavy they are, but surprisingly only about twice as much as lithium if 48v 20ah.)
Any further thoughts about the cable etc?
Thanks

 

[woodman-46]

PS The agm batteries I am buying are deep cycle ones.

 

[JohnW2]

Yes I didn't think it would be a very long time but enough for my young son to have a bit of fun in a paddock etc, the most ah possible for a small 12v agm battery is 15ah by the looks of it.

Fair enough.

Any further thoughts about the cable etc?

Well, 800W is jsut under 17A, so you would be looking for a cable intended for a very minimum of about 20A. However, to make the most of your limited battery capacity it would make sense to minimise voltage drop in the cable, so I would be inclined to go for something 'rated' for currents appreciably higher than that. There's no downside of getting 'too big', other than size, stiffness and cost. I would think that your best bet is probably to get cable designed for automotive use (i.e. go to a 'car shop').

To give you a little feel, you used, say, standard 2.5mm flex ('rated' at 25A), you would get a voltage drop of about 0.3V in a total length of 1 metre with the motor at 'full pelt', which I think would probably translate to a 'loss of power' of about 5%.

However, all that is 'off the top of my head'. Others may have more specific ideas/advice.

Kind Regards, John

 

[ericmark]

The wheel chair adaptor which is to help a person pushing the chair so motor does not steer the chair had a 20 Ah VRLA battery. It may help on hills but it would cause a strain to steer the chair on pavements with a camber and nearly every pavement next to a road has a camber so it was not a good tool, however clearly you can get 20 Ah batteries, however 2 x 12 Ah batteries are claimed to give a mobility scooter a range of 12 miles, so I suppose you are looking at around 24 miles range for 4 batteries, at 4 MPH that is 6 hours use.

Clearly higher speeds will reduce time and range, but although rated at 800W one would hope it would be much less than that most of the time.

 

[woodman-46]

Thanks again. I am planning now, after your advice, on using 33amp cable that is about 3mm thick and keeping this as short as possible between each connection.
I will need to buy a 48v charger to charge the batteries and would use one similar to the photo and also the socket in photo. Looking at the diagram of the four 12v batteries which terminals do the charger cables attach to if they are to be left in place?
Also if I am going to put a fuse in the general set up should it be placed in one of the cables actually going to the motor ( one of the two cables on the right on diagram) and what would be an ideal fuse for this?
I appreciate your help so far.
James

 

[ban-all-sheds]

Talking of starting currents and fuses in EVs...

https://www.wired.com/2015/07/teslas-new-ludicrous-mode-makes-model-s-supercar/

 

[John D v2.0]

To be honest at 800W I'd be more worried about falling off/taking off than it only lasting an hour!

Also if possible it's best to charge as 4x 12v batteries rather than charging them all together. Or ideally 24x2v cells. That avoids differences in the cells causing incorrect charging.

 

[ban-all-sheds]

To be honest at 1 BHP I'd be more worried about falling off/taking off than it only lasting an hour!

?

 

[gblades]

Are you going to have each battery separate or stick them all together as one unit?
If you are sticking them all together then I would be tempted to just use earth bonding wire with suitable insulation.

What charger are you using? For 4 batteries in series you need 13.8*4 or 55.2V. I don't know how easily you will get one of those. Maybe ones designed for fork lift trucks etc... but they are probably going to try and charge too quick. A cheaper way would be to get an old PC power supply and feed the 12V output into a DC-DC converter. Something like https://www.ebay.co.uk/p/Dc12-60v-t...rter-Module-BQLZR/2115795113?iid=253050213919 and you can adjust the voltage trim pot to about 55V and set a suitable current limit.

 

[John D v2.0]

?

it would help if you put some effort into making your point.
I certainly would need a very long uphill or a strong headwind to spend an hour on a bike producing 800w.

 

[ban-all-sheds]

This is a converted motorbike with a 1hp engine.

My point is why on earth would you be worried about falling off or taking off on a bike with only a 1hp engine?