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Stilllearning

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This is a question that probably has been answered before, but if it has, I’ve not seen it.

We have a Rapido motorhome, with an engine start battery, and one domestic battery. They are linked via a 240 volt battery charger that’s built in to the vehicle, and I’m guessing here, by another circuit so that they are both charged when the engine is running.

We want to add a solar charging option, to charge both batteries. Any suggestions of a way to do it, and how much solar panel power to fit?

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3 minutes ago, Stilllearning said:

This is a question that probably has been answered before, but if it has, I’ve not seen it.

We have a Rapido motorhome, with an engine start battery, and one domestic battery. They are linked via a 240 volt battery charger that’s built in to the vehicle, and I’m guessing here, by another circuit so that they are both charged when the engine is running.

We want to add a solar charging option, to charge both batteries. Any suggestions of a way to do it, and how much solar panel power to fit?

It will normally have a split charger relay, or at least my mates does, thats for charging from the engine. His engine battery however isnt connected to his battery charger, and when we added solar we ignored it then. It seems who ever made his campervan, made it so  the leisure battery ran everything inside the van including his dash radio and cab lights. Its a coach conversion and seems to be well put together apart from the heater!! it was diesel but he has ripped it out and replaced it with a gas system

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As Peterboat  says just connect the solar to the domestic battery (using a suitable fuse close to the battery). For the vast majority of time the start battery will be well charged UNLESS you run things like the vehicle lamps, heater, car radio etc with the engine turned off. If you want solar to charge both banks then the split charge relay would need changing for a bi-directional VSR Voltage Sensitive Relay).

 

 

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Our old boat Grebe was wired as Peter describes.  The caravan/motorhome charger was wired into the load/vehile side of the domestic isolator switch, so we could also run the domestics with no domestic battery fitted.

 

The electrics gurus will have an answer (or more).  My only thought is that any connection to the engine battery needs to be independant of the domestic battery to preclude any chance of draining the engine battery by domestic services.  probably all taken care of by the controller but may need a diode if not.

 

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We have a red arc split system on ours.

if the engine battery is over 13.6 v it charges the domestic , if the domestic is over 13.6 it charges the start. I simply put the solar on the engine battery to ensure start up. You do not want a flat battery in rural australia.

the only problem is the relay going berserk on a sunny morning as the start battery dumps into the domestics weary from running the fridge all night and lowering the volts below 13.6 , only to restart the process. This doesnt  last long.

 

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6 minutes ago, peterboat said:

It will normally have a split charger relay, or at least my mates does, thats for charging from the engine. His engine battery however isnt connected to his battery charger, and when we added solar we ignored it then. It seems who ever made his campervan, made it so  the leisure battery ran everything inside the van including his dash radio and cab lights. Its a coach conversion and seems to be well put together apart from the heater!! it was diesel but he has ripped it out and replaced it with a gas system

The battery charger on ours charges both batteries. The evidence for this is at the control panel. Two tell-tales, each digital, each showing over 13v when plugged in to the mains.

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44 minutes ago, Tony Brooks said:

As Peterboat  says just connect the solar to the domestic battery (using a suitable fuse close to the battery). For the vast majority of time the start battery will be well charged UNLESS you run things like the vehicle lamps, heater, car radio etc with the engine turned off. If you want solar to charge both banks then the split charge relay would need changing for a bi-directional VSR Voltage Sensitive Relay).

 

 

Yes, but.

We have a motorhome with an automatic gearbox, and my paranoia makes me want to know 100% that I can always start the engine in the morning.

We have a small (6watt) solar panel that does nothing when connected to the engine start battery. I bought it in a weak moment, at Argos.

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Well, I can guarantee that one day the start battery will fail eventually and nowadays the usual failure method is an internal short that you will not know about until one day it won't start.  As an example I started my car without any signs of a problem, drove to Eastbourne (say nearly 2 hours), stopped an had coffee, started gain with no trouble and drove to B&Q , bought my bits and it would not start because the battery had failed. There is nothing you can do to stop that sort of thing.

 

At least you still have the domestic bank and, as on a  boat, both bank's negatives should be joined by a thick cable so a single decent jump lead between the two bank's positive should get it started. Most "leisure" batteries used for domestic batteries are in fact start battery construction with handles so it won't do any harm.

 

Dead right, 6Watt solar panels will only ever be battery maintainers and I doubt even that when the get old and increase their self discharge rate.

 

If you want solar to charge both batteries then connect it to the engine battery and change the split charge relay for a VSR sensed off the start battery. That way whenever a charge source increases the engine battery voltage to above 13.6 to 13.8 the VSR will join both banks. You might even consider changing the charger connections to the engine battery a swell so that charges both banks when the voltage is above 13.6 to 13.8 from whatever source.

 

 

How much solar depends upon what you can afford and how much electricity you use but for a camper I would suggest 100 Watts will do but maybe no if you are a heavy user of electricity and do not connect to the 240 volts or drive about each day.

 

 

  • Greenie 1
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You don't need a split charge relay to charge both battery banks off a solar panel, this can be done via the outputs from a suitable solar charge controller, which you must have anyway as part of your system.  Just connect direct from the controller to the batteries with suitable thick cable and fuse.

 

On the last motorhome I had a 100w solar panel and a dual battery PWM solar charge controller from Photonic Universe.  Would have preferred a bigger panel but there wasnt room on the roof.

 

On my boat I have 2 x 90 watt panels with an MPPT controller which provides a main charge to the domestic batteries and a trickle charge to the engine battery.   MPPT controllers are the ones to go for, they are the most efficient, but until recently only charged a single battery bank, hence the earlier PWM controller.

 

Have a look at the very helpul FAQ section on Photonic Universe website.

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3 minutes ago, Onewheeler said:

Why?

Yeah, I wondered that too. Ours is a Euro 4 engine, 3 months newer and it would be Euro 5. We found this out by asking Mercedes Benz by email, and were amazed to get a phone call from Stuttgart, from a nice German guy apologising that our motorhome was just too old to get into the London Emissions Zone.

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22 hours ago, Tony Brooks said:

Well, I can guarantee that one day the start battery will fail eventually and nowadays the usual failure method is an internal short that you will not know about until one day it won't start.  As an example I started my car without any signs of a problem, drove to Eastbourne (say nearly 2 hours), stopped an had coffee, started gain with no trouble and drove to B&Q , bought my bits and it would not start because the battery had failed. There is nothing you can do to stop that sort of thing.

 

At least you still have the domestic bank and, as on a  boat, both bank's negatives should be joined by a thick cable so a single decent jump lead between the two bank's positive should get it started. Most "leisure" batteries used for domestic batteries are in fact start battery construction with handles so it won't do any harm.

 

Dead right, 6Watt solar panels will only ever be battery maintainers and I doubt even that when the get old and increase their self discharge rate.

 

If you want solar to charge both batteries then connect it to the engine battery and change the split charge relay for a VSR sensed off the start battery. That way whenever a charge source increases the engine battery voltage to above 13.6 to 13.8 the VSR will join both banks. You might even consider changing the charger connections to the engine battery a swell so that charges both banks when the voltage is above 13.6 to 13.8 from whatever source.

 

 

How much solar depends upon what you can afford and how much electricity you use but for a camper I would suggest 100 Watts will do but maybe no if you are a heavy user of electricity and do not connect to the 240 volts or drive about each day.

 

 

Thanks Tony, much of what you say helps, and makes sense. I will see if there is a wiring diagram for the vehicle anywhere online, or from the manufacturer.

 

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3 hours ago, Onewheeler said:

Why?

Because they have 'smart' charging systems where the alternator charges the battery to less than 100%, relying on regenerative braking to do the rest. Adding a second battery confuses this. The accepted method of charging second battery is via a B2B charger.

 

If you want more info then somewhere I have chapter and verse. But glad I'm retired from vehicle electrics now.

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23 minutes ago, pearley said:

Because they have 'smart' charging systems where the alternator charges the battery to less than 100%, relying on regenerative braking to do the rest. Adding a second battery confuses this. The accepted method of charging second battery is via a B2B charger.

 

If you want more info then somewhere I have chapter and verse. But glad I'm retired from vehicle electrics now.

Thanks for that, I have often wondered why the ECU controlled alternators could tell if they had another parallel bate try connection and still don't fully understand why or how but that answer makes some sense. I am not sure a vehicle without electric drive motors can do regenerative breaking unless they only bring the alternator on line during breaking.

 

Amen to you last paragraph.

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9 minutes ago, Tony Brooks said:

Thanks for that, I have often wondered why the ECU controlled alternators could tell if they had another parallel bate try connection and still don't fully understand why or how but that answer makes some sense. I am not sure a vehicle without electric drive motors can do regenerative breaking unless they only bring the alternator on line during breaking.

 

Amen to you last paragraph.

 

 I think Pearley meant to say the alternator is controlled by the ECU to do most of its charging on a trailing throttle, or overrun, rather than regenerative braking, thus saving fuel and reducing emissions.

Edited by cuthound
Clarification
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On 31/08/2019 at 15:35, Stilllearning said:

This is a question that probably has been answered before, but if it has, I’ve not seen it.

We have a Rapido motorhome, with an engine start battery, and one domestic battery. They are linked via a 240 volt battery charger that’s built in to the vehicle, and I’m guessing here, by another circuit so that they are both charged when the engine is running.

 

 

I'd be surprised if the two batteries are "linked" by the charger? The charger would most likely have two separate 12v outputs but they'd be isolated from each other.

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6 hours ago, blackrose said:

 

I'd be surprised if the two batteries are "linked" by the charger? The charger would most likely have two separate 12v outputs but they'd be isolated from each other.

Good point, so we will have to look at either two solar panels, or a way of linking our two batteries up for solar charging.

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On 31/08/2019 at 17:39, malp said:

You don't need a split charge relay to charge both battery banks off a solar panel, this can be done via the outputs from a suitable solar charge controller, which you must have anyway as part of your system.  Just connect direct from the controller to the batteries with suitable thick cable and fuse.

 

On the last motorhome I had a 100w solar panel and a dual battery PWM solar charge controller from Photonic Universe.  Would have preferred a bigger panel but there wasnt room on the roof.

 

On my boat I have 2 x 90 watt panels with an MPPT controller which provides a main charge to the domestic batteries and a trickle charge to the engine battery.   MPPT controllers are the ones to go for, they are the most efficient, but until recently only charged a single battery bank, hence the earlier PWM controller.

 

Have a look at the very helpul FAQ section on Photonic Universe website.

I have been meaning to get one of these for our sail boat:

1267087127_upload_documents_1600_640-bluesolar-duo-12-24v-20a-front.jpg

Edited by rusty69
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4 hours ago, rusty69 said:

I have been meaning to get one of these for our sail boat:

1267087127_upload_documents_1600_640-bluesolar-duo-12-24v-20a-front.jpg

This Victron is a PWM controller, you might want to have a look at these MPPT controllers before deciding.  I have one on my narrowboat, and it has breathed new life into the 10 year old batteries that came with the boat 2 years ago.   The engine start battery only needs a maintenance charge, which this provides.
1476359170_12208100.png

 

Edited to show CORRECT links!

Edited by malp
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5 minutes ago, malp said:

This Victron is a PWM controller, you might want to have a look at this MPPT controller before deciding.  It is the controller I have on my narrowboat, and it has breathed new life into the 10 year old batteries that came with the boat 2 years ago.   The engine start battery only needs a maintenance charge, which this provides.
1476784433_05250100.png

Thanks.Looks good, though the blurb suggest this too is a PWM controller.Though a PWM would probably suit my needs.

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10 minutes ago, Stilllearning said:

So, with 2 roughly 120Ah batteries, how many watts of solar panelling is considered necessary for maintaining the batteries at an optimal level of charge?

Depends on your usage and camping style.If you do many miles and plug in at a campsites, a small panel 50W is probably all you need. If you do a lot of wild camping without mains or generator, then something bigger, than 100W.

 
We have a 50W panel on the yacht (previously 27W, but I busted it a few weeks back).It did a great job of maintaining a single battery for 7 years. When on board, we put the big panel out (120W) to cope with day to day charging.
 
Lots of flexible panels on ebay of the 100W to 150W would probably suit.
 
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