Use of an Inverter as a Split Charger

wildebus

wildebus

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This is something I have wondered about for a while as an alternative to a B2B charger.

This is the theory... You have a decent smart Mains Charger already you use on hookup - say for arguments sake a 40A one (could be anything).
A 40A Battery to Battery Charger costs around the £200 mark minimum. But you can get a basic 500W Inverter for around £30.
If you had a 20A Mains Charger you could get away with a 300W Inverter - the basic rule being the inverter has to be able to provide the maximum current the Mains Charger can run at.

So you have the split charge cable you would have anyway going to an inverter, then you have the inverter output going to the mains charger you would usually have on hookup.
Ok, you are going 12V to 240V and back to 12V so there is energy wastage in the conversion for sure, but that wastage is taken care of by the Alternator (so maybe a touch worse fuel economy?) however the money saved on a B2B Charger should more than compensate.


Anyone tried this kind of thing
 
If you value your batteries, the important thing is that one smart charger should be in charge.
It's no good having a charger that carefully runs through the four charging phases if half way through the process, some other charger muscles in with a different agenda.
Indeed, that is exactly what a split charge relay or voltage sensitive relay will do when you start the engine.
 
It's an interesting theory isn't it.

If the inverter was powered from a relay on an ignition feed it would only run when the engine was on.

A big saving would also be made in my case because I would not need to run expensive 50mm 12v cable from the cab to my rear garage where the batteries and charger would be sited. The dedicated inverter could be at the front and some 2.5 twin and earth or similar would work nicely to get the mains to the back.

Sounds a bit Heath Robinson, but it could work.

In my case, as this would be the only form of charging (apart from hookup) there would be no vsr etc to interrupt it.
 
One thing that would worry me, would there be a safety issue in the event of an accident where the engine didn't cut out?

Probably very unlikely, but just a thought.
 
Chris Evans001 said:
It's an interesting theory isn't it.

If the inverter was powered from a relay on an ignition feed it would only run when the engine was on.

A big saving would also be made in my case because I would not need to run expensive 50mm 12v cable from the cab to my rear garage where the batteries and charger would be sited. The dedicated inverter could be at the front and some 2.5 twin and earth or similar would work nicely to get the mains to the back.

Sounds a bit Heath Robinson, but it could work.

In my case, as this would be the only form of charging (apart from hookup) there would be no vsr etc to interrupt it.
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I have never tried this myself, but have no doubt it would work. In order to check this I ordered a little 500W Inverter from Amazon today and will try this out in the next few days just to see what happens. There are definately a double set of losses, but that is a penalty that may be worth paying.

Ref the inverter position, a common location for the EHU plug in the VW Transporter conversion world is under the bonnet, so if the Inverter were also under the bonnet, it would be super-easy to wire this up as well. (you would need to protect it from damp though which could be a problem?)
Using a relay with the control signal run by ideally the D+ signal could remotely and automatically turn the inverter on and off with the engine (I actually sell a remote relay control kit for Inverters but with a Switch - that could easily be changed from a switch to a D+ signal)
 
I would install under the front passenger seat (Iveco daily) so damp wouldn't be an issue.
I'll look forward to the results then!

The website looks extremely smart by the way. Will definitely be ordering the relay kit. Not to switch an inverter, but to isolate a switch panel. Also the radio kit that can take an additional supply from a leisure battery looks very handy.
 
Chris Evans001 said:
I would install under the front passenger seat (Iveco daily) so damp wouldn't be an issue.
I'll look forward to the results then!

The website looks extremely smart by the way. Will definitely be ordering the relay kit. Not to switch an inverter, but to isolate a switch panel. Also the radio kit that can take an additional supply from a leisure battery looks very handy.
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Cheers :) If you wanted any customised/specific wiring to suit, just let me know (y)

Ref the Inverter theory... because I have a 100A VSR fitted anyway, for the purposes of the test, I will be disconnecting the output of the VSR from the leisure battery and connecting to the inverter so it will auto self-power when the engine starts up (a slight cheat to make the testing easier, but won't effect the testing results). An similar way to do this is to use a cheap lowish-power VSR like a TEC3M to control a relay. For an outlay of under £10 it is probably worth it to save the hassle of tracking down a D+ signal.

For my Camper, this is just a test of a concept as I have a B2B already, but it I did not, I would be considering this very much as it can be such a waste to have multiple Smart Battery Chargers (in my own Campervan, I have a Smart Solar Controller (aka Charger), a Smart Mains Charger and a Smart B2B Charger - 3 clever and expensive items and only one is in use at any one time :( )
 
My hobby a mk7 Transit with the engine running it’s a constant 14.3 volts not sure if hobby fitted anything with the transit being smart charge I will get my mate to connect his fluke dc clamp on amp meter see what it’s putting out
 
Chris356 said:
My hobby a mk7 Transit with the engine running it’s a constant 14.3 volts not sure if hobby fitted anything with the transit being smart charge I will get my mate to connect his fluke dc clamp on amp meter see what it’s putting out
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Interesting.
typically, the problem with an alternator used for battery charging is the voltage does not typically go high enough to do a full charge. At a constant 14.3, potentially you could be putting on an excessive voltage if the battery was fully charged and should really be at float (~13.8V) (this is why 'proper' intelligent AC-to -DC or DC-to-DC chargers are always best where possible).
Could take a long time when driving to get to a float condition so I would not be surprised if there is a B2B type charger fitted there and you maybe haven't noticed or been running on a single journey long enough to see it drop?

This is the classic charging pattern for a battery (top line is Voltage, bottom line is Current)

Victron MPPT Charging Scheme by David, on Flickr
the Voltage in ABS would be around the 14.3V/14.4V for most Lead Acid batteries and is in that state for a long time often.
 
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So ... Will it work and is it efficient enough to bother? Testing Time :geek:

To get the benchmark, I plugged the Mains Charger into the mains to see what the batteries will ask for currently at their current state of charge.

Output from Charger on EHU by David, on Flickr
So the full 20A it can provide the batteries will take.

Some quick theroretical maths ... maximum current is 20A, maximum voltage will be 14.4V. So maximum power out is upto 288W. Throw in charger inefficiencies of say 20% and the maximum power drawn in could be around 350W from the mains supply.
If we plug this into an Inverter instead, that will have its own inefficiences - let's call that 20% as well. so for an inverter to supply 350W, it will need to pull in 420W - or in 12V terms, upto 30A (reckon on the alternator running at 13.8V typically, so it is 420W/13.8V) .
So potentially, to run a battery charger putting 20A in at upto 14.4V, the alternator needs to supply upto 30A.
That is perfectly doable I think. But what are the real-life test figures? :unsure:

I bought a basic 500W Inverter from Amazon
  1. connected the 12V supply side to the cable that comes from the alternator to the split charge setup;
  2. connected the 240V output side to the 20A Victron Mains Smart Charger I have.
  3. connected the 12V output side to the battery bank
So we are going DC ---> AC ---> DC

This is the output from the Mains Charger

Current - Charger to Battery by David, on Flickr
(ignore the minus current - it is just the way I had to position the meter to get to see it)
Putting out 21.53A in fact (as it can put out 20A at 14.4V, it is obviously capable of more at a lower current then).
The Leisure Battery Voltage at this point is 13.15V

This is the input to the inverter from the Alternator

Current - Alternator to Inverter by David, on Flickr
The Inverter is pulling in 25.49A
The Starter Battery Voltage (so same as Alternator) is 13.79V


In terms of Power,
Mains Charger is putting in 283 Watts (21.53A x 13.15V) into the Lesiure Battery Bank
Alternator is putting out 351 Watts (25.49A x 13.79V)
That works out to be an overall efficiency of 80% in the charging process which is actually very decent (I expected this to be a fair bit worse in fact)

Overall Conclusion? I reckon if you have a decent mains charger already in place, and you were thinking of getting a B2B charger from when driving, it is worthwhile considering the above method. It could give you a substantial saving in money and deliver the same results :alien:

A very important note: If you did try the above, the Inverter you get MUST be capable of delivering the maximum power demand of the Charger within its continuous rated voltage. It is the Charger that determines how much power is called for, NOT the Inverter.
In my test, the Charger will never need more than 350W and the inverter I bought was rated at 500W continuous, so a good match there.


If anyone wonders, for this test this is the Inverter I bought - https://amzn.to/2CI0gNl. It was selected on a very simple basis ... the cheapest 500W Inverter on Amazon with next day delivery. With the £5 promo code it was under £21. It seems pretty decent however the cables supplied were a bit too light and the croc clips were worse than useless. So should anyone buy one of these, factor in having to get new cables (but that is typical of most inverters, sadly)
 
That's very interesting. Assuming you have a good mains charger, it looks like a smart idea. Switch the inverter on with a relay from the D+ wire and all should be good and automatic, as long as the charger will not go comatose if its mains supply goes off.
How many people do have a good mains charger? I have a powerful mains charger and the one built into the van is a fairly good smart one, but not very powerful. Not an issue for me, because I so rarely use hookup.
 
hairydog said:
That's very interesting. Assuming you have a good mains charger, it looks like a smart idea. Switch the inverter on with a relay from the D+ wire and all should be good and automatic, as long as the charger will not go comatose if its mains supply goes off.
How many people do have a good mains charger? I have a powerful mains charger and the one built into the van is a fairly good smart one, but not very powerful. Not an issue for me, because I so rarely use hookup.
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Good point about the type of charger. My charger 'auto-starts' when it gets power with no user interaction needed. I know some don't, but any mains charger that is suitable for campervan/motorhome use at all really needs to be the type that self/auto starts on power being supplied I would say.

I have a 40A B2B charger already so there is little benefit to me to change, but if I had not already had this, I would look at the Inverter option very seriously.
 
I have a smart charger that needs a button pressed (twice) when the mains comes on. An utter pain. I'd not have bought it if I'd known. I just use it for looking after spare batteries in the house.
But I was also thinking about the smartness as well as the autostart abilities.
A proper multi-stage charging cycle is vital to stop your batteries being cooked, and if a smart charger gets to somewhere in the balancing stage (for example) then gets its mains power removed, will it know where it was up to when its power is restored?
A good B2B charger does remember, because it is designed that way. I don't know which mains chargers can do this.
 
hairydog said:
...
A proper multi-stage charging cycle is vital to stop your batteries being cooked, and if a smart charger gets to somewhere in the balancing stage (for example) then gets its mains power removed, will it know where it was up to when its power is restored?
A good B2B charger does remember, because it is designed that way. I don't know which mains chargers can do this.
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When you refer to the Balancing Stage, are you talking about Equalising between cells in a battery by applying a higher voltage from the charger?
I did have one B2B Device - the PI/25C Power Integrator from Amperor Associates - that used to do an automatic equalise process on every 10th charge, but I don't think most B2B chargers will do this unless instructed (if at all). Certainly none of the Mains Smart Chargers I have used will go into this mode without being set to it specifically and it is generally not recommended to automatically do an equalize on a regular basis unless you suspect you have a problem or you have a certain battery type (I think the Trojan T105s like a regular equalise for example?).
So this is not something I personally would be concerned about as I very rarely enable that feature and if needing to do so would 'choose my moment' to do so when I could be pretty sure of uninterrupted power for the 60 minute or so duration (be it on EHU or Driving)
 
Hi, i found this thread while googling after having the same idea! So glad to see somebody has actually tried this and found it to work well, My only thoughts now are how could i wire this up so that my battery charger ran from my inverter charges my leisure batteries while my engine is running but at the same time i still want to be able to use my inverter for other 240v items when the engine is off while being ran from my leisure batteries and not on hook up, Would this be possible?

So if i connected my inverter to a split charger coming from the main starter battery then while the engine is running i could charge my batteries off the inverter with a charger connected however when i switch the engine off the split charger will turn off and thus i would be unable to use the inverter. Is there a way to connect this up so that i can run the inverter from my leisure batteries while the engine is off? The only thing that comes to mind is the idea of connecting the inverter to both the starter battery via a split charge relay but also to my leisure batteries via a manual battery isolator that i could connect once the engine is off and disconnect once the engine is running but that seems like a bit of a ball ache and also what would happen if i forgot to disconnect from the leisure batteries while the engine was running?

Is there another simpler way to go about this? I already have a 1500 watt inverter and 20a charger, seems like a waste to buy a b2b charger aswell.
 
Bassman said:
Hi, i found this thread while googling after having the same idea! So glad to see somebody has actually tried this and found it to work well, My only thoughts now are how could i wire this up so that my battery charger ran from my inverter charges my leisure batteries while my engine is running but at the same time i still want to be able to use my inverter for other 240v items when the engine is off while being ran from my leisure batteries and not on hook up, Would this be possible?

So if i connected my inverter to a split charger coming from the main starter battery then while the engine is running i could charge my batteries off the inverter with a charger connected however when i switch the engine off the split charger will turn off and thus i would be unable to use the inverter. Is there a way to connect this up so that i can run the inverter from my leisure batteries while the engine is off? The only thing that comes to mind is the idea of connecting the inverter to both the starter battery via a split charge relay but also to my leisure batteries via a manual battery isolator that i could connect once the engine is off and disconnect once the engine is running but that seems like a bit of a ball ache and also what would happen if i forgot to disconnect from the leisure batteries while the engine was running?

Is there another simpler way to go about this? I already have a 1500 watt inverter and 20a charger, seems like a waste to buy a b2b charger aswell.
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I’ve been doing this for years. I have one inverter running from the engine batteries that I can plug a charger in for the leisure batteries. I also have an inverter running of the leisure batteries and another charger I can use to charge the engine batteries. And all the other stuff you want to do.
 
To use a single inverter on both supplies, you could use a pair of high power relays - one connected to the starter battery, another connected to the leisure battery. both outputs going to the Inverter.

next step - use a low power 5 pin inverter (the typical 40A 12V relay). Wire up as follows:
Pin 30 connected to +12V
Pin 85 connected to GND
Pin 86 connected to D+ from the Alternator
Pin 87 connected to the Starter Battery High Power Relay pin 86
Pin 87a connected to the Lesiure Battery High Power Relay Pin 86

Set it up this way, when the engine is off, the inverter is powered by the Leisure Battery; when engine on, by the Starter Battery.

One Gotcha ... when you do this, you end up powering the Mains Charger by the Leisure Battery - not a good idea!
To fix that, get an 240V SSR fitted between the Inverter and the Mains Charger, connected the +12V control connection on the SSR to the Low Power Relay Pin 87. That way the Mains Charger only gets power from the Inverter when the engine is running (and SSR will be enabled), otherwise the SSR is off.

You can also use a 240 Auto-Relay in that circuit, so when you have mains hookup, the supply to the mains charger is diverted to the EHU.
Picture of setting up a Auto-Relay and SSR to follow.

That is how I would set it up.
 
This is how you could setup the switching for the mains charger.
The "To Boiler & Fridge" would be To Charger
The "From MCB2" would be "From EHU"
The "From MCB0" would be "From Inverter"
The "Control from Cerbo Relay" would be "From Pin 87 Low Power Relay"
1605559002306.png
 
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