 | Home > Technical > Is it possible to trickle charge into a dual battery system? |
 
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| donmacn Member Since: 06 Nov 2017 Location: Nth Scotland Posts: 1845  
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Hi folks,
I did search, honest, but couldn't find anything that seemed to me to be answering this question. I can crimp wires and connect things together neatly, but I know my own limitations - and really understanding batteries and electrickery is one of them. I have a 1994 300Tdi. It has a dual battery set-up - connected via a Voltage Sensing Relay.  Click image to enlarge Leisure battery now at 6.5v after about 2 months of not being used; the car battery was at 10.5 and I'm hoping that the charger (connected in the pic) will bring it up to a useable level, even if only for a short while. The leisure battery has a solar charge input (via a controller and the VSR) and also has an Anderson SB connector, on a 10A fuse, which can be used to power accessories. For convenience sake I fitted a little panel of three Anderson connectors in the 2nd row footwell (no seats as it's just 2 of us). The solar panel plugs into one of them when in use; the fridge into another; and there's the 12v Acc. one. Click image to enlarge My ideal scenario would be to be able to plug a 240v mains charger into that 'accessory' Anderson connector to charge or trickle charge both batteries at the same time, while still connected to the car wiring. i.e. without having to disconnect the batteries from the car which is a bit of a chore. By my simple logic, charge would then flow towards the leisure battery, and at some point the VSR would connect and allow charge to get to the car/starter battery. Is there any sense to this? Would any charger or trickle charger work this way? In case its relevant, I'll be changing the batteries anyway. At the moment the car has Optima AGM batteries fitted, but these just don't last for me. I've read on here that it might be something to do with these batteries requiring charge characteristics that the older fashioned alternators don't supply. I'm aiming to find 2 x EFB or standard wet batteries that will fit crossways in the battery box under the passenger seats. The Optimas are very good for size - but that's pointless if they don't last. My query is definitely about the charge/trickle charge options - not looking to revist the debate about AGM batteries. 4 expensive batteries in 5 or 6 years is just not sensible. As always, any advice gratefully received. Donald 1994 Defender 300Tdi 110 SW - owned since 2002 - 230k miles and going strong (The 'rolling restoration' or tinkering thread: http://www.defender2.net/forum/topic58538.html ) 2000 Range Rover P38 4.0L V8 in the past.. RR classic - fitted with 200Tdi 1984 RR classic - V8 with ZF auto box 1993 Discovery 300Tdi not to mention the minis and the Type 2 VW camper... |
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8th Dec 2024 6:19pm |
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| donmacn Member Since: 06 Nov 2017 Location: Nth Scotland Posts: 1845
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Thanks Bluest.
I'm able to follow your first two paragraphs - and what you've said makes sense. I'd have to check the specs of my chargers. One is a more modern 'Ring' one; the other an older Halfords job - but not ancient. For your third para, I'm not sure where the sensing lead might be, and in fact I think my system is a bit more complicated. I have a Smartbank & gauge system fitted and this benefits from being permanently connected to allow the software to more accurately track the state of charge of the batteries. I think this means there isn't one sensing lead - but that the Smartbank electronics does this. I've emailed the suppliers to see if they might say anything about this. This recommendation to keep the batteries connected as much as possible is why I was hoping to just plug in one charger to the existing wiring. (That said I've had to disconnect them today anyway!) Maybe these problems will just go away when I buy better batteries! Donald 1994 Defender 300Tdi 110 SW - owned since 2002 - 230k miles and going strong (The 'rolling restoration' or tinkering thread: http://www.defender2.net/forum/topic58538.html ) 2000 Range Rover P38 4.0L V8 in the past.. RR classic - fitted with 200Tdi 1984 RR classic - V8 with ZF auto box 1993 Discovery 300Tdi not to mention the minis and the Type 2 VW camper... |
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8th Dec 2024 7:50pm |
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| Mdm Member Since: 11 Sep 2013 Location: Sunny Lancashire Posts: 1619 
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you can do this but you need a bigger enough amp charger to fll the battery's.
eg red top need a charger of xxx amps ( info is on their web page) i have the same system on the 90 and also have both battery's wired with c tek connectors so i can charge them 1 at a time if the aux on is flat or super low. on my other car i have the std twin battery set up and a leisure battery and charge it off one 12 amp smart charger. no issues. |
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8th Dec 2024 8:06pm |
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| macfrank Member Since: 05 Nov 2015 Location: somewhere in the north Posts: 1081  
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At 6.5V the battery is probably toast and probably also the other one. Best to be checked with a battery tester such as the Motopower MP0515A.
You could try reconditioning. I'd recommend the CTEK MXS 10. Only 5 Amps isn't up to job as Bluest said. But better replace them first before overthinking other possible issues. I've had no problems for the last 9 years using a Varta Dual Purpose EFB as starter and an Ective Deep Cycle Gel as aux battery, both connected by a VSR and (trickle) charged by a MXS 10 which is permanently installed in the Defender. (Only) if one or both are really down, the on/off cycle Bluest described occurs. I have a switch in the ground wire of the VSR, disabling it (there is no separate sensing lead on mine). Each battery has a CTEK plug, so I can charge them separately if needed. |
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8th Dec 2024 8:29pm |
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| jst Member Since: 14 Jan 2008 Location: Taunton Posts: 8049 
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I have several split charge systems in different vehicles.
Using an older high amp charger no problems when vsr joins batts. With my mx5 charger i tend to plug it in straight after a drive when both batts showing good xharge anyway, no problems on this state. You could just fit another lead between the two batts with a switch. Connect them together and charge both by bypassing the vsr. Vsr works both ways whether charger is on aux or primary doesn't matter. Cheers James 110 2012 XS Utility 130 2011 M57 bespoke Camper 90 2010 Hardtop 90 M57 1988 Hardtop |
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8th Dec 2024 8:46pm |
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| donmacn Member Since: 06 Nov 2017 Location: Nth Scotland Posts: 1845
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Thanks all for the replies - appreciate that. I'll look at all those recommendations.
I'm absolutely reconciled to the fact that I need two new batteries.  One of my chargers does have a 'repair' function, so I might pull the batteries from the car and try that on them one at a time to see what happens - but I don't know if that charger will hit the voltages needed for Optimas. It has a setting for lead acid, but not specifically AGM. That's an example of something that I simply didn't know: that the Optimas needed higher charge voltages. The first set were professionally installed/selected, so when they died I just replaced like with like. Twice bitten.... there won't be a third time. Or maybe it's just operator error - that might be a fair call.
I helped a friend install a split charge system into his car a couple of years ago before a Pyrenees trip, using 2 EFB batteries from Tayna, and he's had no issues at all. His car probably spends more time un-used than mine. I think that's what I'll be going for. However Bluest's post made me think I needed to check or remind myself exactly what the SmartBank/Gauge is doing. I researched all of this before installation - but that's about 2 or 3 years ago and I've since forgotten it all. I've been doing a 'deep dive' into the manual and I think its functions mean I wouldn't end up in the VSR doing a connect/disconnect cycle. Among other things its specifically designed to properly share the input of one charging source across two or more batteries. The manual also says this: "Do not expect a SmartGauge - SmartBank combination to operate as a simple voltage controlled relay. It is far more sophisticated than that. SmartGauge takes over 30 measurements and makes tens of thousands of calculations every second." Perhaps I should have checked that first - but if the system doesn't behave itself as I think it should, I'll be looking closely at some of your suggestions. Donald 1994 Defender 300Tdi 110 SW - owned since 2002 - 230k miles and going strong (The 'rolling restoration' or tinkering thread: http://www.defender2.net/forum/topic58538.html ) 2000 Range Rover P38 4.0L V8 in the past.. RR classic - fitted with 200Tdi 1984 RR classic - V8 with ZF auto box 1993 Discovery 300Tdi not to mention the minis and the Type 2 VW camper... |
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8th Dec 2024 8:54pm |
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| Julie Member Since: 07 Oct 2017 Location: Nantes Posts: 496 
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Why don't you just disconnect the service battery before you connect the charger ? 
After all a 12V below 11.8 needs replacement |
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9th Dec 2024 7:19pm |
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| Julie Member Since: 07 Oct 2017 Location: Nantes Posts: 496
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Why don't you just disconnect the service battery before you connect the charger ?
After all a 12V battery below 11.8 needs replacement |
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9th Dec 2024 7:19pm |
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| donmacn Member Since: 06 Nov 2017 Location: Nth Scotland Posts: 1845
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Thanks Julie,
Of course I can disconnect either battery. In fact that's what I've done at the moment as I try to see if I can eke a little more life out of the starter battery. Who wants to buy 2 new batteries in the weeks before Xmas!? But as I said initially what I was hoping to find out was whether, in normal use, I could simply plug one mains charger into the Aux. connection on the outside of the battery box for convenience sake. ( I think, with my smartbank gadget I can, but that's still to be confirmed). Donald 1994 Defender 300Tdi 110 SW - owned since 2002 - 230k miles and going strong (The 'rolling restoration' or tinkering thread: http://www.defender2.net/forum/topic58538.html ) 2000 Range Rover P38 4.0L V8 in the past.. RR classic - fitted with 200Tdi 1984 RR classic - V8 with ZF auto box 1993 Discovery 300Tdi not to mention the minis and the Type 2 VW camper... |
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9th Dec 2024 8:32pm |
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| Naks Member Since: 27 Jan 2009 Location: Stellenbosch, ZA Posts: 2645  
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I have a dual battery system (National Luna), and I leave my Puma on trickle charge for 3-4 months at a time when we're overseas.
I had the sparky put a charging port from the main battery to the battery box, and I just plug in my Ctek there. I have noticed that when the main battery is full(ish), the solenoid kicks in, and then the secondary battery also gets charged. -- 2010 Defender Puma 90 + BAS remap + Alive IC + Slickshift + Ashcroft ATB rear 2015 Range Rover Sport V8 Supercharged
Defender Puma Workshop Manual: https://bit.ly/2zZ1en9 Discovery 4 Workshop Manual: https://bit.ly/2zXrtKO Range Rover/Sport L320/L322/L494 Workshop Manual: https://bit.ly/2zc58JQ |
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10th Dec 2024 7:44am |
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| Julie Member Since: 07 Oct 2017 Location: Nantes Posts: 496
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Add a 12V DC2DC charger to your Xmas wish list
I have this config and my 5 amp charger charges : Either starter battery and service battery at the same time Or service battery alone + care function And I can connect a 200 W PV panel to the DC2DC charger which charges first the service battery and then the starter battery |
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10th Dec 2024 8:35am |
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| JackNorris Member Since: 16 Mar 2020 Location: Milton Keynes Posts: 70
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I have a tmax split charge and have the same issues described above and end up on the circle of solenoid in, solenoid out
My current solution is to manual link batteries before charging My future plan is to put a relay in so that when ctek is connected it triggers the relay which joins the batteries and bypasses vsr Haven’t thought about it too much yet |
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10th Dec 2024 10:57pm |
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| macfrank Member Since: 05 Nov 2015 Location: somewhere in the north Posts: 1081
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A relay to connect both batteries when charging seems a good idea, consider, however, that it needs to be able to carry higher amps than the maybe 10 amps from the charger. If the batteries' voltages are different, current will flow between them until balanced. I measured upto 40A, but that's just an example for my setup and specific voltages. This current may only be that high for a few seconds or minutes, but chose a relay which can handle that current (they're not expensive). Also proper wire size.
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11th Dec 2024 12:22pm |
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| landy andy Member Since: 15 Feb 2009 Location: Ware, Herts Posts: 5724 
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I like simple. So just attach the a jumper wire between the two batteries when charging them. The idea of a relay is far too complicated for something rarely done. The wire from the charger is only small so as long as it is the same size or bigger it will work.
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11th Dec 2024 12:30pm |
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