Ebike battery as motorhome leisure battery

[Bman80]

Hi tech minded people.

I am converting a lwb boxer minibus into a motorhome and am considering using my ebike battery for the leisure battery. Tell me if you've done it/how you'd do it/if I'm crazy.

It is primarily for running a 12v compressor fridge, which I am yet to buy, but will consume an average of ~1A @12v... So 24aH in motorhome speak or 288Wh in pedelec. I plan to install a rigid rooftop solar panel to provide for this consumption and some extra capacity for 3 season camping.

I spent some time reading about vehicle leisure batteries and then thought, hang on I've got a really good li-ion on my e bike, can I use that?

My bike is a trek 7.3fx with a woosh aikema rear hub kit and has a 17.5Ah 36v battery. By my calculations this is 630Wh, sufficient to power my fridge for a couple of days.

The issues I see are...
1, voltage -36v battery and fridge demanding 12 or 24. I can see DC-DC "buck" converters on eBay which claim 95% efficiency for converting to 12/24v from higher. These only provide 240 to 480w which might be a problem providing the high amps for starting the compressor, so I might need a power capacitor on the 12v side ‍

2, charging... Decent Solar chargers are made for charging li-ion batteries, so shouldn't be a problem. Perhaps a second solar charger regulator for charging from the alternator is better than using a 240v inveter and supplied charger block.

Any thoughts appreciated...

 

[vfr400]

A 100Ah 12v leisure battery costs £80. Chargers and converters, etc. for your ebike battery will probably cost more, make a much more complicated system with less capacity and more restrictions on your ebike usage. KISS!

 

[Bman80]

Ha ha, I do hear ya.

That was my plan at the beginning.
But turns out an £80 100Ah battery can only utilise half it's capacity without being damage so has 50Ah@12v=600Wh, same as the bike battery. An £80 lead acid is only rated for 80 charge cycles, so might only last a year or two. My mate bought 2 of these last year, for the same purpose as I've described and they barely hold charge any more.

As I looked for something better I ended up looking at spending more like £200. I'd still need the solar kit and converter for off grid anyway, so the extra costs would be the DC converter @£10-£20,a cradle £11 and possibly a power cap @£30.

 

[Benjahmin]

No answers here but an interesting ponder.
The reason that Li-ion car batteries do so well is that the charge/discharge cycle depth is very restricted compared to bike batteries. That's to say that the management sytem only allows charge to around 80% of cell capacity (whilst still allowing cell balancing)and a discharge to about 20% of capacity.
Whereas a bike battery (to save weight and size) is charged to 100% of cell voltage (utilising much simpler bleed voltage balancing) and discharged much lower before low voltage cutoff. This deep cycling shortens life.
If you could get your solarpanel to provide more than 24hrs draw during daylight you might be able to 'float' the battery. However cell balancing would suffer and not sure if Li-ion like being floated..
As far as I'm aware, leisure batteries are specifically designed to be deep cycled (unlike standard car batteries) and lead acid is well suited to being floated

 

[Bman80]

Yes, interesting point about restricting charge on Li-ion car batteries. I did wonder why my battery was rated for 800 cycles whereas cars manage many more.

I have a 24kWh Nissan Leaf. The actual capacity of the battery is 22kWh limited by the BMS.
It has been recharged several thousand times over 5 years and still retains 93% of this original capacity.
It also has a "long life mode" which charges to 80% of the nominal battery capacity.

I understand that modern solar controller controllers are designed for charging Li-ion batteries, as these are becoming more common as leisure batteries. However, I don't know how they deal with the charging cycle and floating etc.

 

[Woosh]

I don't have a motorhome but I find the subject very interesting.
What we need for our e-bikes is a bit on the margin so e-bike batteries are more expensive than the average price of Lithium-ion batteries.
What would be the ideal weight, voltage and capacity would you like for your motorhome?

 

[Bman80]

Hi Tony, loving the Aikema kit, BTW. Though not put many miles on it as I commuted once, then Covid has got me working from home ever since. Have cycled up several mountain passes with 20kg of toddler on the back though which is much more fun - especially with no cars about.

I'm not sure that the e-bike batteries are priced any higher than the purpose made 12V Lithium-ion leisure batteries I've seen online:

This 50Ah/ 640Wh battery is £470

12V 50AH Relion Lithium ion Battery RB50 - Alpha Batteries

12v 50 AH Lithium ion deep cycle battery RB50 Hailing from the USA, RELiON is the latest addition to our […]

www.alpha-batteries.co.uk

So similar in energy capacity to the Woosh 17Ah 36V battery that I have and costs £399.

However their voltages and battery management are obviously tuned for vehicle leisure battery use (some detail of voltages and charge rates in the datasheet on that link). I think the resting voltage of a regular 12V vehicle battery is 12.7V.

Obviously I'd love a large 12V capacity weighing very little and very cheap please...

Its easier to define my minimum requirements:

As in my calculation of the first post I need to draw an average of 1A @12V to run the fridge (1A is generous- some people have reported half this figure). So using full capacity of my battery would run it for about 2 days, which is fine for weekends away.

I plan for longer trips next summer and would install sufficient solar capacity (~200W) to top the battery up during the daytime. In which case the battery simply needs to get me from 3pm to 9am, 18Ah or 216Wh. It could in that case be run with a smaller capacity cycle if such control were available.

 

[Woosh]

if you only need to run the fridge and the TV, it makes sense to use the e-bike battery for that instead of buying a new one.
You would need a cradle for your HL battery (I can sell you one), and a 36V-48V to 12V step down converter like this one:

Rungao Voltage Reducer Converter Regulator DC 36V/48V To 12V 10A For Golf Cart : Amazon.co.uk: Automotive

Rungao Voltage Reducer Converter Regulator DC 36V/48V To 12V 10A For Golf Cart : Amazon.co.uk: Automotive

www.amazon.co.uk

 

[Bman80]

Thanks, that's exactly the step down converter I've had my eye on.

Unfortunately I think my problem is going to come from the solar charger... I have asked photonic universe to recommend me a solar charger which is capable of charging a 36v li-ion battery. The only one that they supply is the VICTRON SMARTSOLAR MPPT 150/35 35A, which is £320.

Without the requirement for 36v charging I would only have to spend £100 on a solar charger, so it's this component that breaks the bank in the end. Shame.

 

[peter.c]

Thinking side ways what happens when they replace the battery in a electric car? have seen a few packs on ebay cheap will that provide loads of cells in a rack mount the prius packs look cheap

 

[Nealh]

Thanks, that's exactly the step down converter I've had my eye on.

Unfortunately I think my problem is going to come from the solar charger... I have asked photonic universe to recommend me a solar charger which is capable of charging a 36v li-ion battery. The only one that they supply is the VICTRON SMARTSOLAR MPPT 150/35 35A, which is £320.

Without the requirement for 36v charging I would only have to spend £100 on a solar charger, so it's this component that breaks the bank in the end. Shame.

About 3 years ago I stopped and spoke with a guy on his ebike and he had an array of flexible solar panels laid out to charge his 36v batteries, from what I could tell he had a simple looking small inverter /charger and a watt meter to show wh input, voltage etc,etc. I forgot the details but he said he was able to keep doing so for his touring needs, most of the panels folded flat or rolled up to stow on his Heath Robinson rack he made and two panels were mounted like a rear aerofoil spoiler permanently trickle charging a spare battery carried.

 

[Woosh]

one cheap solution is to use the solar panel to charge your car battery then use a 230V car inverter to charge your battery when the sun is out.
The 230V car inverter does not need to be anything special, your battery charger uses about 100W - your solar panel can cover that plus conversion loss.

Streetwize Power Inverter

For powering up mobiles, laptops and other appliances from a vehicle battery. Converts 12V DC vehicle battery to 230V AC to provide 300W continuous power. Can be powered through a 12V socket when using appliances below 150W.For users that need access to an electrical supply when out on the road...

www.toolstation.com

 

[Nealh]

I use a small solar panel for my car, it plugs in to the cigarette lighter to trickle the battery. A diode is used to prevent reverse charge.