UKH

I am planning to get an inverter for our campervan so I can charge the E-bike battery from the leisure battery. The 13A plug on the charger will go into the 230V output socket of an inverter. I am assuming from the specification that came with the bike that I will be able to do this as the handbook says "Voltage and Charge Current Cc: 42V/2A" which seems to me to say that the battery is drawing 84 watts. (I don't know what Cc in the foregoing means). Or have I got this wrong? If I'm right that is the case then quite a small inverter would do the job. 

Next: I would be happy to run the inverter off the cigarette lighter socket to save having the thing hard-wired into the van. What is the max amps which can be safely drawn through a cigarette lighter circuit?

Lastly: if anyone has experience in this field I'd welcome advice on which inverter to choose, or indeed to avoid.

Yes - 42v/2A will be 84 Watts from the charger to the Ebike battery. The charger won’t be 100% efficient converting from 230V to 42V, but will be in the high 90’s. The inverter from 12V to 230V similarly won’t be 100% efficient. Draw from your vehicle battery could be close to 100W (~12.5V/8A). Cigarette lighter fuses vary from 5A to maybe 20A, but most likely is 10A. Might be worth checking the fuse in the fuse box. If it is 10A this will be fine for the charger.

What amperage is your battery.

If it’s the 6.9 amp then you’ll need 6.9 x 36 volts to charge it.

That’s 248 watts to charge your battery from empty.

If your leisure battery is 100 amp then you shouldn’t discharge it below 50% unless it’s a deep cycle battery.

So your leisure battery holds 100 x 12 = 1200 watts.

50% of that is 600w and you only need 248 to fully charge your bike battery so that’s OK.

Of course that’s less power available to run your other electronic equipment.

Sorry, I know bugger all about inverters and their efficiency.

Buy or borrow a cheap AC power meter to find out what the charger actually uses - that’s the best way to find out.  Efficiency losses mean it’ll draw more power than it puts in to the battery, and labels aren’t always right...

If it charges at ‘w’ Watts for ‘h’ hours you need ‘w*h’ spare capacity in Watt Hours in your leisure battery.  Again I’d go for a value measured by a power meter in preference to a calculation. I’m guessing it’s a few hundred watt hours - which could be enough to tip a single leisure battery (likely around 1 kWh) over from “happy” to “sad” with all its other loads.

’Cc:’ potentially refers to the constant current phase of charging.

Inverters - beware “modified sine inverters” - that’s most of the really cheap stuff - as a lot of switch mode power supplies (which the charger basically is) will barf on these and either not work, become very inefficient or make funny noises.  You want a “pure sine” inverter.

Depends!  In practice I’ve found them to be a bit erratic for continuous, higher power use and depending on the quality of the plug and the socket and the connection between them.  Sometimes the tip of the plug can be hot enough to hurt skin after half an hour’s extended use with a draw under 10 A.  They’re normally fused at 10 A or 20 A.  They’re a really inappropriate design for continuous high power draw, I certainly wouldn’t sleep in a van with one being used for that sort of thing...

Well done for properly distinguishing between power (in watts) and energy (in watt hrs).

I don't understand the statement higher up the thread that a leisure battery holds 1200 watts. It might hold 1200 watt hrs.

Batteries store energy not power. They can subsequently supply this energy at a particular power. Comparing the energy stored in the bike battery with the energy stored in the leisure battery provides some guidance. It might mean getting a second leisure battery!

Sorry if this sounds like a rant. I used to believe that all should understand the 2nd law of thermodynamics and evolution by natural selection before they left school. Now I think that understanding the difference between energy and power (together with their units) would do.

Colin

Indeed.  Note that the energy stored in a lead acid leisure battery is such a fluid concept...

https://en.m.wikipedia.org/wiki/Peukert%27s_law

Disccharging at 100 W to run the bicycle battery charger, the leisure battery probably has a lot less capacity than its sticker says.  The good thing about getting a second battery is you halve the per-battery discharge and therefore increase their capacity under Peukert’s law and so more than double your total capacity.

If I wasn’t bothered by budget, I’d go for a lithium ion battery in the camper - much closer to the sales sticker capacity at realistic discharge rates and you can discharge them almost totally without degrading them unlike lead acid ones.

It’s because I failed my physics O level.

Watts, watt hours, amps, they’re all double dutch to me.

I did, however, attempt to be helpful and answer the OP’s question, unlike you.

If you by your own admission have very little understanding of battery physics, perhaps you shouldn't be trying to answer questions on batteries.

I actually gave the OP an answer based on years of personal experience.

The fact that I used the wrong term to describe a unit of energy had no real affect on the validity of my answer except in the mind of a pedant.

Rather than charging from a leisure battery, you might do better charging directly from a solar panel, if you have one: this is my brother-in-law's solution. I'm assuming you might have, seeing as you are talking about a leisure battery and not talking about EHU. Charging directly from solar means you don't run the risk of discharging your leisure battery and possibly damaging it, though this does require the sun to shine of course.

Well I did suggest an additional battery might be useful which I thought was relevant to the original question.

However you are in good company. The business and energy articles of most newspapers  reveals that most journalists are similarly confused. " This backup battery provides 10 MW of grid storage is a typical statement." There is no indication whether 10 MW can be provided for a second, minute, hour or day. Is it pedantic to point this out? I do think the issue is relevant to the original question particularly if a lot of money is going to be spent.

Quite a few useful points here including the actual energy from lead acid batteries as a function of power (rate at which energy taken) and the possibility of solar. I think you really have to crank through the numbers for some of them. Leisure batteries (lead acid) don't mind being run down but don't necessarily like to deliver high power. Lithium batteries might be a good alternative (often used in small power packs to charge mobile phone batteries) but you need to understand the cost per kWh storage. For solar panels you need to understand what area you need and take in to account the solar intermittency - not so good for charging up overnight. I agree that cigarette lighter sockets might have limitations for high currents over a long period. They seem to need to cool down between one cigarette and the next though they are fine for mobile phone charging. Obviously charging up your e-bike battery while driving makes some sense. It does all depend what your anticipated need are.

I find it good fun to come up with a solution for this sort of thing so good luck. It would be interesting to read about your experience with anything you implement.

I'm glad you've cleared that up (the watts and watthour business). I was thinking there was something wrong.

We do have quite a large solar panel on the roof of the van. In addition the leisure battery is topped up by the van's alternator. I've been thinking if all else fails we could charge up while driving or on a clear sky day.

Tell that to NASA who lost a two hundred million dollar spacecraft due to a mixup over units, or the passengers and crew on the legendary “Gimli Glider” who inadvertently set the world record for longest unpowered flight in an airliner...

Units may seem like a pedantic point but they’re critically important.  I don’t chime in on posts with unit errors on UKC but I will chime in to support important pedantry.

Comparing Rog Wilko’s e bike to a NASA spacecraft are we?

While it is really critical if you mix up units while filling your aircraft with fuel does it really matter to most people if a leisure battery has 1200 watts or watt hours, especially when most batteries are advertised by their amps?

It may well have more computer power than an Apollo spacecraft did.  Caution is always wise when doing sums for battery work - even a puny little backup AGM out of a house alarm can happily start fires or vaporise a lot of stuff if you guff it.  A leisure battery can do all sorts of serious mischief... 

Yes.  Incorrect units forcing people to make assumptions about what they think are the correct units leads to mistakes.  

In terms of the validity of an answer - the rules where I am are that if marking an answer with the “correct” number and the incorrect (or missing) units is to award 0 marks for the answer.  Because the number isn’t correct.  The number in isolation is meaningless - the united answer is the answer. Ergo, wrong units wrong answer.  Luckily most marks are awarded for legible, explained workings.  This comes as a shock to A-level students who are trained Pavlovian style to scribble a bunch of numbers with no explanation or units, and to write a purely numeric answer in a box with the units pre-printed next to it...

They’re normally advertiser by their “cold cranking amps”, a rather quaint term for “most amps you can draw for 10 seconds without wrecking it” and their “amp hours” as a capacity rating.  Sometimes I look at my bank with its ~3000 CCA rating and think about having some fun...

I might be rubbish at understanding basic physics but it didn’t stop me welding a screwdriver to my bike frame when I accidentally short circuited my e bike battery.  

Apologies if short circuited isn’t the accepted term for what I did.  

Very good. A fair amount of power (energy delivered in a small amount of time) required to do this.

It seems some entrepreneur could usefully design an e-bike battery charger which is properly connected to the vehicle electrical system, has a battery docking station, minimises conversion losses and is used when the engine is running. Buying a second e-bike battery rather than a second leisure battery might then make sense.

There is of course a fair amount of discussion about this on various e-bike forums.

One rather faffy way of doing it with this setup on a cloudy day is to keep an eye on the leisure battery voltage and disconnect the eBike charger when it drops below about 12.2 V and reconnect it when at least ten minutes have passed and the battery goes above 13.5 v.  This way you’ll on/off cycle the charging when there’s some but insufficient solar so that the leisure battery stores and releases solar harvested energy without cycling to the point it’s going to start degrading.  If you get an inverter with an enable input you could use an arduino to do it.  I’d have thought charging for one hour in three would have much the same effect.

What’s your solar charge controller?  PWM or MPPT?

I was understanding until you got to this bit.

At what voltage?  We have banks of backup lead-acid batteries at work - 3x 100Ah strings @54V per system. I've seen a video of two or three of these systems parallelled up and shortcircuited.  The ~1" diameter 'welder cable' hanging down in a nice curve instantly jerked straight before the insulation in the whole system spontanteously combusted and everything caught fire :-O.  A colleague accidentally shorted between a terminal and the earthed rack with his watch one one system - it burnt all the back of his hand as he slid it off.

In case anyone is interested, here's some feedback on the inverter question. I bought a pure sine wave inverter (300 watt output) for about £60 and had it fitted by the local auto electrician. Currently enjoying a Pyrenean holiday in the van and finding no problems charging the bike battery with just the leisure battery with solar panel in the background. The lowest voltage the leisure battery has reached is 12.5 v. Thanks for all the advice and suggestions.

Rog

This is what I do at work. I put a 309w inverter in my van to run off the lighter socket in the back and use it to charge my drill battery's and my impact gun batteries. The socket is only live when the engine is running but that's not a problem because its only charging a spare battery anyway. In my last van I hard wired it and forgot to turn it off on a long weekend, flat battery on Tuesday morning.