Possible to power a 12v battery tender using ebike battery ?

[Frankie55]

Hi All not sure if this is the right place but I’m trying to figure out a way to use an ebike battery to keep my car battery from going flat, I’ve got a 52v to 12v step down converter could I use a charge controller with this to trickle charge the lead acid ?

Thanks in advance

Frank

 

[amberwolf]

Wouldn't it be cheaper and simpler to just get a car-battery trickle charger that plugs into the wall?

(since you'll have to plug in the ebike battery's charger into the wall and into the ebike battery, to keep it from going flat while it's charging the MUCH higher capacity car battery via the low-efficiency multiple conversions you'll have).

Would also be less wear and tear on the (often expensive, limited lifespan) ebike charger and battery pack.


If you do need to use the ebike battery for some reason, you'll need a converter from the ebike battery voltage level to somewhere between 13.6v - 14.4v (check your car battery label for it's actual charging voltage).

12v "automotive" (including motorcycle, etc) systems are not actually 12v (or rather, that's the nominal voltage since they call a lead acid cell "2v" and there are six of those in each lead-acid car battery...but they're actually going to be about 13.6-14.4v or higher for charging and when they are full, depending on exact type. If yours is not a lead-acid but is a more modern lithium replacement type, it will be made to match these levels, and should also be labelled with the actual charging voltage it needs).

 

[john61ct]

Charging from storage, one battery from another, with no actual charge source present

is incredibly wasteful, energy inefficient

and is wearing out two batteries at once rather than just one.

You would need a very unusual use case to justify doing so.

Why do you think you need to do that?

 

[calab]

Well some chargers do work off of 12v (dc) source, lots of remote controlled hobby chargers for example.
The higher power systems people use server power supplies because they can handle 12v 60a (700w) and you can put them in series with the modification, 24v 60a (1.5kw), 36v 60a (2.2kw), 48v 60a (3kw) you can also play with the fan speed and fan noise.

One technically can power a 120v ac battery charger (Tender) with a 12v dc battery, you'd just need an inverter from 12v dc to 120vac to plug the battery charger into to charge whatever battery you want to charge, but its not very wise.

You can also buy an inverter for you car, so technically your using your car battery to charge whatever battery you want. Same idea its just a matter of your use case. The more stuff there is, the more problems that can occur so the less stuff is better and more reliable.

 

[amberwolf]

That is the opposite of what the OP wants to do.

They want to charge a 12v car battery from a higher voltage ebike battery.

Well some chargers do work off of 12v (dc) source, lots of remote controlled hobby chargers for example.
The higher power systems people use server power supplies because they can handle 12v 60a (700w) and you can put them in series with the modification, 24v 60a (1.5kw), 36v 60a (2.2kw), 48v 60a (3kw) you can also play with the fan speed and fan noise.

One technically can power a 120v ac battery charger (Tender) with a 12v dc battery, you'd just need an inverter from 12v dc to 120vac to plug the battery charger into to charge whatever battery you want to charge, but its not very wise.

You can also buy an inverter for you car, so technically your using your car battery to charge whatever battery you want. Same idea its just a matter of your use case. The more stuff there is, the more problems that can occur so the less stuff is better and more reliable.

 

[john61ct]

Well some chargers do work off of 12v (dc) source, lots of remote controlled hobby chargers for example.

As do the DC-DC chargers from Sterling, Redarc, Renogy etc.

The docs may suggest they are "battery to battery" but in reality they are powered by an alternator or solar, wind hydro etc.

 

[jonyjoe303]

most buck converters (which is what a step down converter is) can be used as is to top off a 12 volts battery. By design they are CC/CV which will good to charge any lead acid.

I use buck converters to keep my van start battery topped off. But on mine I use a LED volt/amp meter so I know how many amps are going into the battery and disconnect it when its full.

 

[Frankie55]

I do have a trickle charger but the car is parked up where there is no sockets, and I have a spare ebike battery I could use just to prevent car battery discharge

Would it be possible to use the 52v to 12v step down into something like a solar charger controller for lead acid ?

I already have an inverter but it runs a fan for cooling which would affect the efficiency unless I could disable the fan, I imagine the load will be very small supplying the trickle charge to a full battery

Something like this

https://www.ctek.com/uk/cs-free-portable-charger-and-power-bank-12v-for-your-vehicle-battery

 

[Frankie55]

I already use a small solar panel to the car battery but not much sun here in Ireland haha

 

[john61ct]

So get a bigger panel. Really doesn't take much unless you have other problems

A disconnect switch to isolate the battery from whatever's trying to run it down.

Or maybe your battery just needs replacing?

If that high voltage pack is any good it is worth many times more, best to take care of it rather than risk wearing it for such a trivial use case