Load capable battery charger for AGM leisure

[StevieRuss]

Hi guys.

First post here and it's a bit of a lengthy technical one.

I've kitted out a camper with a 100watt solar panel (putting another up soon) feeding an epever 20amp solar charger controller that feeds 2 x 120ah AGM batteries in parallel. I've also fitted a split charge relay.

The batteries are constaantly responsible for powering a Webasto 12v CR65 fridge (when out and about) as well as the usual 12vdc fuse boxes I've installed to power lights and USB chargers and the like. The battery capacity and solar panel do a great job of maintaining a high voltage at the leisure batteries but on occasion i"d like to employ the hook up to feed a proper battery charger (AGM compatible) to condition/charge/load the batteries and more to the point provide a current suitable for the load on the batteries.

I know this probably means a load capable charger (not sure what I'm looking for here but probably 20-30amp something or other). The idea is that the consumer unit is currenlty powered by a 1000watt Pure Sign Wave inverter which in turn feeds 2 x 13amp AC sockets. I have installed a switch that swaps the feed from the inverter to the hookup so the 13amp 240vac sockets are fed from the hookup only however I'm wondering if I could take an isolated feed from the hookup and wire it directly to a proper battery charger and the use the system that already exists to power the van (i.e. Hook up only fed to the charger).

I know this sounds like a round about way of doing things but the leisure batteries are running what I've designed as an off-grid van which I'm happy with. I don't essentially need the hookup unless I've been static for a while in which case it would be nice to "solely" use it as a feed to a suitable battery charger which would then be happy with a variable DC and AC (inverter fed) load.

If anyone bothered to read this far then thanks. Anyone who gets what I'm doing and knows more about such chargers then please do let me know!

Thanks in advance. Regards. Steve

 

[ProfJohnL]

Hello Steve.

Yes I have read it, and I think I understand your approach to the issue.

Technically it wold work but, my first though was how inefficient your use of mains power (Hook up) would be.

When charging batteries, you can sometimes use twice as much power as the power you will get out. It depends on the type of battery, the type of charger, and the condition of the batteries, the amount of power you are trying to extract etc...So I can't give yo an exact figure.

On top of that you using a pure sine wave inverter, (Which I applaud for teh safety of your appliances) but they have intrinsically higher losses 30 to 40% than a modified sine wave inverter (10 to 15%).

So putting it another way if you were using 1000W of ac power in the caravan, you would need to have a supply of up to about 2500W.

I think it would make better sense to have your incoming hook up 230V ac supply feed a more modest 200W multistage charger, which in turn charges your batteries in conjunction with your solar panels. But to have your internal 230V socket connected to a properly rated 13A change over switch with one side fed from your inverter or the other side fed from the 230V in coming hook up. This could be fairly easily be arranged using a relay which will automatically change over when ever the Hook up power is available.

 

[VicMallows]

I agree entirely with the Prof on this one.

I would like to emphasize though that if you go this route you MUST ensure you have a properly designed, well engineered and foolproof change over system for the 230v. (I can assure you that even a few milli-seconds of mains on the output of your inverter will destroy it!).

 

[StevieRuss]

Hello Steve.

Yes I have read it, and I think I understand your approach to the issue.

Technically it wold work but, my first though was how inefficient your use of mains power (Hook up) would be.

When charging batteries, you can sometimes use twice as much power as the power you will get out. It depends on the type of battery, the type of charger, and the condition of the batteries, the amount of power you are trying to extract etc...So I can't give yo an exact figure.

On top of that you using a pure sine wave inverter, (Which I applaud for teh safety of your appliances) but they have intrinsically higher losses 30 to 40% than a modified sine wave inverter (10 to 15%).

So putting it another way if you were using 1000W of ac power in the caravan, you would need to have a supply of up to about 2500W.

I think it would make better sense to have your incoming hook up 230V ac supply feed a more modest 200W multistage charger, which in turn charges your batteries in conjunction with your solar panels. But to have your internal 230V socket connected to a properly rated 13A change over switch with one side fed from your inverter or the other side fed from the 230V in coming hook up. This could be fairly easily be arranged using a relay which will automatically change over when ever the Hook up power is available.

Thanks for your quick reply!

I suppose that the inverter was actually always going to confuse the issue a little. I put it in, not to run tv's, computers and other household appliances on the whole, but to offer some 240vac when required. This is literally going to be a hand blender on occasion (I love cooking!) as well as charging my laptop. I have popped a 8watt 240vac bedside light from home in there recently as it's a little light on the 12vdc lighting at this stage, which will eventually provide all or almost all the lighting we will use. The inverter was just one of those things I decided to put in so although inefficient at very low current draw, it didn't really factor in as effecting the off grid experience too much (too much inefficient current draw!). In retrospect I probably should have put in a 500 watt or less to increase the efficiency :huh:

As far as the auto sensing relay is concerned, do you have a link to what you're describing as I am new to the campervan off/on grid wiring stuff (although I work with electrics and lighting for a living)? The manual switch I installed may be fine for occasional use but has had no use as off yet as I am still running the thing as an off grid and just want the dual solution of a proper multi stage battery charger as well as it being load capable.

I've looked at the Victron chargers but they are super expensive. That said, if it fills a gap then it's probably worth it. I suspect there are other suitable solutions out there and maybe cheaper?

Thanks again. Steve

 

[StevieRuss]

I agree entirely with the Prof on this one.

I would like to emphasize though that if you go this route you MUST ensure you have a properly designed, well engineered and foolproof change over system for the 230v. (I can assure you that even a few milli-seconds of mains on the output of your inverter will destroy it!).

That loop mistake would not be fun! I suppose that's one of the reason I wanted to keep the 240ac hook up as purely a feed to a load capable multi stage charger. If the only connection the hook up has is to the charger itself (this is now my main thinking) then everything else I have so far wired, and will wire in to the system in the future is fed from the off-grid side (fuse boxes from the battery and inverter when necessary) and that is already perfectly functional.

There may well be consumption inefficiencies with the idea but it's only inefficient as far as the mains supply is concerned and not an unnecessary drain on the leisure batteries. As far as I know, most hookups are 10amp which is over 2kw which is way more than I need anyway.

Cheers! Steve

 

[ProfJohnL]

Hello Steve.

I must admit I had not given the relay idea very much deep thought at this stage. I could certainly produce a working system, but the problem would be it would not be approved for use on public mains system.

So the idea of a change over switch, or plugs is a more proven and practical a solution and would guarantee a break before make process.

However looking at your problem from a slightly different perspective, essentially you want a battery powered system to take over when mains 230 Vac is not available. This is exactly the same as an uninterruptible power supply or UPS. These are commonly used for computers where a mains failure can have dire consequences. They come in a variety of different power outputs, and they have all the necessary control systems to ensure the switching between grid mains and internal batteries and inverter is properly synchronised to avoid phase and load mismatches.

There could be four down sides to these though, The first is they tend to have a beeper which sounds when the mains power is not present. However I'm sure the beeper could be disabled or removed.

The second is they tend to use built in batteries, which will be not big enough for your needs. But some UPS's use an Anderson connector which means you might be able to use a larger external battery.

Thirdly some use batteries in series - I have an APC 1500VA which uses 24V rather than 12V but I do have 3 other 650VA units which just use 12V.

And fourthly of the 4 different UPS's I have their output is a square wave and not a sine. This is not a problem for purely resistive loads or it seems computers or switch mode power supplies, but it can cause problems with inductive loads such conventional iron cored transformers and electric motors.

If you can get away without needing a Sine wave, then these are distinct possibilities.

Incidentally UK ( and teh rest of the EU mains voltage is now specified as 230V ac not 240.

 

[StevieRuss]

Hello Steve.

I must admit I had not given the relay idea very much deep thought at this stage. I could certainly produce a working system, but the problem would be it would not be approved for use on public mains system.

So the idea of a change over switch, or plugs is a more proven and practical a solution and would guarantee a break before make process.

However looking at your problem from a slightly different perspective, essentially you want a battery powered system to take over when mains 230 Vac is not available. This is exactly the same as an uninterruptible power supply or UPS. These are commonly used for computers where a mains failure can have dire consequences. They come in a variety of different power outputs, and they have all the necessary control systems to ensure the switching between grid mains and internal batteries and inverter is properly synchronised to avoid phase and load mismatches.

There could be four down sides to these though, The first is they tend to have a beeper which sounds when the mains power is not present. However I'm sure the beeper could be disabled or removed.

The second is they tend to use built in batteries, which will be not big enough for your needs. But some UPS's use an Anderson connector which means you might be able to use a larger external battery.

Thirdly some use batteries in series - I have an APC 1500VA which uses 24V rather than 12V but I do have 3 other 650VA units which just use 12V.

And fourthly of the 4 different UPS's I have their output is a square wave and not a sine. This is not a problem for purely resistive loads or it seems computers or switch mode power supplies, but it can cause problems with inductive loads such conventional iron cored transformers and electric motors.

If you can get away without needing a Sine wave, then these are distinct possibilities.

Incidentally UK ( and teh rest of the EU mains voltage is now specified as 230V ac not 240.

Thanks again for your efforts Prof!

We use UPS devices to back up lighting rigs that I work on. More to the point, the lighting desks although at the Olympics in 2012 we had two massive GE UPS units (the size of large scale site generators) that supported the lighting rig at Earls Court during a potential power failure. I'd never seen anything like it at all. Before or since..!

It's a fair point though. The actual switch over circuit is probably the relay system I was trying to get my head around.

The truth is that I am really simply wanted to use the hook-up as a power supply for a decent charger that will run as a power supply for the battery load which again, is quite a low current system.

I was looking at the Victron Blue Energy chargers last night and noticed that they claim their units can be used as a power supply although I can't find mention of whether this is during a battery charge or even having a battery connection. Maybe I am really expecting too much of this idea although I can imagine there are quite a few off-grid occasional hook-up types that have had to overcome similar hurdles.

Many thanks. Steve