Leisure battery advice please?

[SV_WDC]

I need a leisure battery that will be off-grid, connected to a 12v DC to 240v AC converter; connected to a cTEK charger for a car battery

I realise this is poor efficency 12v to 240v back to 12v. Can anyone recommend whether an A, B or C battery is best for this purpose?

And whether there is a way to calculate how many operating hours I'd get out of this setup?

Thanks.

[Panamax]

The question is what you're going to do with this battery.

Car type batteries are designed to be fully charged all the time. An appropriate leisure battery may need to be the "deep discharge" variety that don't mind being constantly flattened and recharged. There's also the question of what capacity you need.

Halfords say,

Class A batteries are perfect for motorhomes or caravans that spend lots of time off grid. They have the highest storage capacity so can support your electrical appliances and equipment for longer before needing to be recharged.

Class B batteries are designed for caravans or motorhomes that spend most of their time hooked up to an electrical supply, but still require a greater battery capacity for certain appliances such as motor movers.

Class C batteries are only designed to be used for short periods away from an electrical supply, Class C batteries have the lowest capacity and can only support basic appliances and equipment when not hooked up, such as LED lighting.

[SV_WDC]

Panamax said:

The question is what you're going to do with this battery.

Car type batteries are designed to be fully charged all the time. An appropriate leisure battery may need to be the "deep discharge" variety that don't mind being constantly flattened and recharged. There's also the question of what capacity you need.

Halfords say,

Class A batteries are perfect for motorhomes or caravans that spend lots of time off grid. They have the highest storage capacity so can support your electrical appliances and equipment for longer before needing to be recharged.

Thanks, I had seen Halfords too and decided type A might be best for my needs.

As for 'what I'll be doing with the battery', it's to power a cTEK charger, plugged into my car. Garage has no power.

Question is how to calculate how many hours use i'd get out of a battery based on this?

[Pedro Raynard]

I'd assume your conversion factor is .85 as start point for each stage so overall 0.72. In practice it will hopefully be more.

How often will the leisure battery be cycled? If the 12v to 240v inverter is running continuously it may be drawing 0.5 Amps continuously on its own. Therefore 100AH battery flat in around a week. Without any charge current to charger. So I would think in 3 days flat battery with a load of another 1 Amp.

I suspect you will need a lithium battery. New Lead acid (leisure) are only any good for half their capacity and would be dead in 12 months with a couple of cycles a week.

Lithium can’t be charged below freezing, some batteries don’t have cold temp protection built in.

You are right to be asking up front, you really need to understand loads and thus size and frequency of charging battery.

I’ve found this website useful, it gives some info in performance of lithium v lead acid. You may need to search the site a little. Other lithium batteries are available.

https://www.lifos.co.uk/lifos-go/lifos-go-105v/

[Panamax]

The problem with attempting that calculation is you don't know how much "work" the CTEK will be doing.

Assume the vehicle on charge never moves.
You need to know the capacity of its battery.
You need to know the condition of its battery.
You need to know how much current is drawn by the car alarm etc.
You also need to calculate/take a guess at the rate of "self discharge" of the battery. i.e. the inevitable loss of charge over time. A good car battery loses about 20% of its charge per month when standing completely idle.
You need to know the efficiency of the inverter. i.e. how much juice gets lost in the process.
You need to know the drain of the inverter when it's doing nothing.

All in all it seems an impossible calculation.

I would buy the biggest capacity Class A battery you can sensibly find and then keep an eye on how quickly it discharges in your use. You'll then know how often it neds to be disconnected and charged.

Edited by Panamax on Friday 24th November 17:02

[SV_WDC]

Pedro Raynard said:

Therefore 100AH battery flat in around a week. Without any charge current to charger. So I would think in 3 days flat battery with a load of another 1 Amp.

Hi Pedro - thanks for the info here. Please can I clarify whether the '100AH battery flat in around a week' is with a charge connected to the leisure battery while it is running?

I am not fastidious about it running 24/7. Ideally I'd like to run a cTEK for 24 hours 1-2 times a week, although it is attractive if I can find a battery that could power the ctek for 3-4 days at a time. In that scenario I'd do this dance once a week in winter, and only once every 2-4 weeks in summer when the car is used with more regularity, and for longer distances.

Edited by SV_WDC on Friday 24th November 17:36

[Scrump]

If you want to use the energy stored in the leisure battery to keep the car battery topped up then would it not be easier and more efficient to just connect the leisure battery straight to the car battery. 12v dc to 12v dc?

Edit:
I see optimate make a dc to dc charger:

Edited by Scrump on Friday 24th November 17:11

[SV_WDC]

Scrump said:

If you want to use the energy stored in the leisure battery to keep the car battery topped up then would it not be easier and more efficient to just connect the leisure battery straight to the car battery. 12v dc to 12v dc?

Edit:
I see optimate make a dc to dc charger:

Edited by Scrump on Friday 24th November 17:11

Thanks Scrump - I think I need to do more reading. I had come across this as a potential option but have been concerned whether it is an adequate way of keeping a car battery topped up with all the other electronics, as cTEK has multi-step charging to preserve battery life.

[Pedro Raynard]

SV_WDC said:

Hi Pedro - thanks for the info here. Please can I clarify whether the '100AH battery flat in around a week' is with a charge connected to the leisure battery while it is running?

I am not fastidious about it running 24/7. Ideally I'd like to run a cTEK for 24 hours 1-2 times a week, although it is attractive if I can find a battery that could power the ctek for 3-4 days at a time. In that scenario I'd do this dance once a week in winter, and only once every 2-4 weeks in winter when the car is used much more.

Think of it this way. I have a 12v to 240v inverter, connected to a leisure battery. With the inverter turned on and nothing connected to inverter, the current out of the leisure battery is 0.5A. Therefore in 200 hours the leisure battery will be flat. If the inverter is turned off, it won’t be drawing any current from the battery and the battery will remain charged.

If you plug something into the inverter, with the inverter switched on, and the current drawn increases by 1.5 Amps, then the total current out of the battery is 2 Amps. So in 50 hours the battery will be flat.

Edited to add. I’m with scrump. DC to DC more efficient.

[ferret50]

Would it not be easier and simpler to use a solar panel to keep the car battery topped up?

Solar panel....charge controller.....battery?

[Harv-surrey]

There a number of 2a trickle chargers on the market that are dc-dc ...

Our system works well for months.

Lesiure battery charged by 30a charger from alternator.
10a solar charger with 200w solar.
2a trickle dc-dc back to vehicle battery.

Then we will shortly be running a 300ah lithium and 1600va inverter for off grid 240v
And adding another 200w solar.

Most things we have run off 12v and there isn't much that needs to be run off 240.