Advice on leisure battery connections please

[offagen]

I'd welcome your informed opinions on using existing leisure batteries with a new inverter please.

Our Auto-Trail has two leisure batteries in close proximity, one in a below-floor battery box and the other in an above-floor battery box. I want to wire them to a new Victron Phoenix 12/1200 inverter which will be close to the above-floor battery. The drawing below shows the existing installation. Battery LB1 is the original leisure battery (below floor) and LB2 is an additional one installed by the motorhome retailer at the time of purchase. It appears that the wiring for the second leisure battery was already provided by Auto-Trail at the time of manufacture. The wiring from both batteries runs to a Sargent EC155 power supply unit via a main wiring harness below the MH floor, so the batteries are connected in parallel at the Sargent unit. The cable length from each battery to the Sargent unit is about 3.5 - 4 metres. The positive wires appear to be 3mm2 and the negative wires 4mm2, i.e. they are nothing like the sizes needed to connect both batteries to the inverter. Those existing cables have to be retained* but can I install additional cables between the two batteries and the new inverter, as shown by the dotted lines on the drawing? Will that then allow the inverter to draw the required current from both batteries without adversely affecting the original wiring or anything in the Sargent unit?

I had in mind 25mm2 cables for the new connections with two new midi fuses F3 and F4 of perhaps 120A rating. Does that sound adequate?

The intention is to change the AGM batteries to two 105AH lithium batteries when finances permit but hopefully the proposed cabling would suit them too.

* The existing cables serve the MH's 12v habitation electrics, connect the solar charger to the batteries and facilitate battery charging (when on EHU), all via the Sargent unit.

 

[Jev88]

Personally I would not involve the Sergeant unit.
My inverter wires direct (fused) to my leisure batteries - Pos on one battery and Neg on the 2nd.
But not knowing much about Sergeant units there may be a reason you want to do it. Are you trying to run All your 240AC items from your leisure batteries? I just plug in the item I want to use off grid direct to the Inverter.

 

[offagen]

Personally I would not involve the Sergeant unit.
My inverter wires direct (fused) to my leisure batteries - Pos on one battery and Neg on the 2nd.
But not knowing much about Sergeant units there may be a reason you want to do it. Are you trying to run All your 240AC items from your leisure batteries? I just plug in the item I want to use off grid direct to the Inverter.

Thanks for your reply Jev88. The reasons for having to retain the wiring to the Sargent unit are explained at the bottom of my post. However, if the batteries and inverter are wired as shown on my drawing it might be possible to disconnect the existing wiring to the second battery, LB2, to avoid any current drawn by the inverter trying to pass through the Sargent.

 

[Speve]

If you are thinking about eventually changing to lithium you might want to consider a Victron Multiplus which has capacity to charge lithium and trickle vehicle battery as your Sargent may not have a lithium setting.

 

[offagen]

Thanks for your suggestion Speve. You are right that the Sergeant unit doesn’t have a suitable charging profile to suit lithium batteries. My MPPT solar charger does and would do the job in good daylight. Unfortunately, I don’t have space to accommodate a 1200VA Multiplus and I already have the Phoenix inverter. I will continue to use my portable NOCO battery charger when necessary, which does have a lithium setting. For lithium I would also need a B2B installed, but that’s another story.

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[Itsmcb]

Your diagram looks feasible to me. You may want to consider the thickness of the new cables depending on the cable length, also you may want to include a second F3 fuse at the LB2 end of the same cable.

 

[Itsmcb]

Useful voltage drop calculator to aid with cable sizing:-

Voltage Drop Calculator | 12 Volt Planet

Use our Voltage Drop Calculator to select the correct Cable for your campervan, motorhome or boat wiring application. Auto and Marine Electrical Components from 12 Volt Planet - Cable Size Calculator

www.12voltplanet.co.uk

 

[offagen]

Thanks Itsmcb, that’s all helpful information

 

[Lenny HB]

With your existing batteries you will only be able to draw 500 watts from the inverter as you shouldn't discharge the batteries at more than the C5 rate which is 21amps per battery for your batteries.
If you want to run it at full power you need to fit another 2 batteries or change to Lithium.

 

[autorouter]

I agree with Itsmcb. With an extra tweek, maybe unnecessary. The reason for using identical length and thickness link cables between the batteries is to ensure the voltage drops are exactly equal. However if one link wire contains two fuses, the total resistance will be slightly more than the one without fuses. So I would consider making the fused link out of thicker wire, maybe 35mm² instead of 25mm².

However, if the batteries and inverter are wired as shown on my drawing it might be possible to disconnect the existing wiring to the second battery, LB2, to avoid any current drawn by the inverter trying to pass through the Sargent.

If the two existing wires are both connected together at the Sargent unit battery terminal, then there will be no reason for anything except maybe a tiny trickle of current to pass through wires to (but not through) the Sargent unit. The worst that would happen would be, if the new F3 inverter fuse blew, then the full current would pass up and down the two thinner wires and would certainly be enough to blow either F1 or F2.

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[offagen]

With your existing batteries you will only be able to draw 500 watts from the inverter as you shouldn't discharge the batteries at more than the C5 rate which is 21amps per battery for your batteries.
If you want to run it at full power you need to fit another 2 batteries or change to Lithium.

Thanks for your advice Lenny. We will have to live with limited inverter output for the time being but, with the present wiring (as shown on my drawing), there is only effectively one battery available to serve the inverter due to the existing long runs of very small gauge interconnecting cables. If I am able to add cables as shown on my drawing I would, at least, gain the use of the second battery - even if still limited to 500w.

The intention is to change to lithium as soon as costs allow but, with the addition of B2B etc, they do mount up. Then obstacles like the eyewatering insurance renewal quote just received from Comfort tend to get in the way so lithium gets pushed back

 

[offagen]

autorouter many thanks for your helpful comments.

I had considered disconnecting the wiring between the second leisure battery (LB2) and the Sargent panel so that no current can be drawn though that circuit by the inverter but my concern then is that the long thin cables of the remaining connection, Sargent to LB1, would be inadequate to carry the battery charging current from the solar controller. It’s a can of worms.

 

[Itsmcb]

With your existing batteries you will only be able to draw 500 watts from the inverter as you shouldn't discharge the batteries at more than the C5 rate which is 21amps per battery for your batteries.
If you want to run it at full power you need to fit another 2 batteries or change to Lithium.

Whilst you are technically correct and I hate to disagree with you, I have been running a 1200 watt inverter and drawing around 100amps off two 110 amp lead acid batteries for the last two years. Admittedly the microwave or (not and) the coffee pod machine are only used for a minute or two at a time but they certainly do work. I do intend to make change to lithium in the long run so I’m not overly concerned about shortening the life of my two batteries but they are still working well