Member Since: 12 Jun 2017
Location: somewhere
Posts: 765
Split charge system - multiple differing AH battery sizes
Bit of advise please
An upcoming 3 day trip to Santa Pod (with no elect hook up ) brought this idea into my head
A split charge batt/ inverter electric hook up for the tin tent. I have never been a fan of generators when camping.
So... the plan is to use 3 different size amp hour batteries, all 3 are different makes.
2nd disco batt - 65ah deep cycle
plus boot mounted battery pack consisting of a 220ah & 75ah dead cycle batts
Basically what i already have laying about, they are all nearly new batts.
All 3 batts are sealed lead acid and will be run in parallel .... so 12v 360ah in total.
which by my math is about 150ah of usable storage, or 10 hours running time for my wattage demands. Then run the disco for an hour or so to top up the batts as and when required.
I then started to read horror stories online about mixing different batteries sizes, apparently they don't charge very well, with the small ones charging faster and discharging more rapidly than the bigger ones.?? I'm struggling to see how myself....
The whole system will be connected by 175amp cable and Voltage sensing relays / fuses.
I do not expect much if any resistance with the wiring.
Granted there may be differing resistance between the varying batts themselves, but surely this is only slight ? Lead acid batteries balance themselves out don't they ?
Obviously this is just for short term use, i plan on charging the batts properly when home again.
So thoughts please.... has anyone done similar with different sized batts ?
Am i going to blow myself up or am i reading far to much into it ?
TIA
11th Mar 2022 1:19 am
Harry365
Member Since: 25 Aug 2018
Location: Lincoln
Posts: 373
Going parallel with different capacity batteries isn't particularly an issue. It's not ideal due to voltage differences between brands etc but as a temp setup it'll work just fine.
You're just adding more cells to a battery already made up of cells.
11th Mar 2022 12:05 pm
jenseneverest
Member Since: 12 Jun 2017
Location: somewhere
Posts: 765
Thanks Harry, that's good way of looking at.
Just got to put it all in now
11th Mar 2022 9:00 pm
Motolab
Member Since: 18 Oct 2019
Location: Sleen
Posts: 1815
Re: Split charge system - multiple differing AH battery sizes
jenseneverest wrote:
which by my math is about 150ah of usable storage, or 10 hours running time for my wattage demands. Then run the disco for an hour or so to top up the batts as and when required.
With a split charge system they won’t be charged well that way…
A DC to DC charger is needed for that…
Andrew will explain it to you (and others) when you want in this video…
Always looking for Pre '55's & Pre war British Motorcycles! knowing or having one for sale? PM please. I visit the UK 6 times a year
Ps. I edit my texts quite often, english is not my native language, so I will edit My “typo’s” etc.
11th Mar 2022 10:48 pm
jenseneverest
Member Since: 12 Jun 2017
Location: somewhere
Posts: 765
very interesting thanks motorlab
What Andrew says makes a lot of sense
never heard of lead crystal batts before, they look ideal for a permanent setup
I have ordered a 150amp shunt and ammeter to see what goes on when charging, will be interesting to see exactly how the disco handles the extra batts.
12th Mar 2022 10:27 am
Harry365
Member Since: 25 Aug 2018
Location: Lincoln
Posts: 373
While Andrew isn't wrong about anything he says, I think he has exaggerated how much of an issue it is. As he points out, the main battery loves to to discharge it's stored energy and providing you have a decent cable between main and aux, there's no reason the aux battery won't take advantage of this and take a decent draw when the relay is open. D3s are also blessed with a 15.2v alternator which really helps things along (this also restricts the batteries you can connect without intervention).
Split charge is definitely not the perfect solution but it's cheap and effective. By all means go for an ideal set up but if you're looking for a quick fix, split is the way to go. At the end of the day, like most things, it comes down to money and time.
Even if the non ideal scenario shortens the life of your battery, at least the cost is spread over years, VS an expensive one off purchase followed by a slightly longer battery replacement interval. For some people, this is a big consideration.
12th Mar 2022 10:36 am
jenseneverest
Member Since: 12 Jun 2017
Location: somewhere
Posts: 765
exactly that, cost is the biggest factor, that and i cant see it been used more than about 3 times a year.
If it were not for the batts and pure sine wave inverter already laying about i probably wouldn't have bothered. I'm still about 200 quid out of pocket for cabling and other bits and bobs...
A few bags of ice for the cooler box would probably have worked out a bit cheaper lol.
12th Mar 2022 11:26 am
UKKiwi40
Member Since: 01 Mar 2020
Location: London
Posts: 12
Hi Harry
If a split charger systems isnt the ideal are you saying a DC DC charger system is?
This is an area of my setup Im planning currently and looking for a similar sized system.
Brendan
12th Mar 2022 11:41 am
Harry365
Member Since: 25 Aug 2018
Location: Lincoln
Posts: 373
A DC DC charger is one step up from split chat but still has its draw backs. It excels in the way it pulse charges your leisure batteries however there are still limitations. They typically charge at around 20 amps, potentially a lot less than a split charge system that can easilier handle 100amps plus. They also suffer from the same activation issues that split charge relays suffer from, is they only turn on at X voltage and turn off at Y voltage.
For the money, I seriously don't see the benefit.
In my opinion, the ideal setup is a second alternator with solar. I believe the tdv6 can accommodate a second alternator though I've not seen it done. I had plans to replace the water pump pulley with a dual belt pulley and mount alt via engine lift bracket. Never got round to it and happily running my split charge system for 7th year. In 7 years I have replaced the batteries once, not because they were damaged from charging but because I ambitiously put a 2000watt inverter on them, drawing 160amps for 5 mins at a time, several times a day for 2 years. My split charge system maintained decent charge on these batteries in this use case.
The comprise best system would be a split charge relay and solar. The solar with charger would give you the advantages of DCDC charger in terms of charging technology.
12th Mar 2022 12:01 pm
Motolab
Member Since: 18 Oct 2019
Location: Sleen
Posts: 1815
UKKiwi40 wrote:
Hi Harry
If a split charger systems isnt the ideal are you saying a DC DC charger system is?
This is an area of my setup Im planning currently and looking for a similar sized system.
Brendan
I am of the DC DC charger camp…
Used split charge in my D3 with a yellowtop and a Snomaster classic fridge, was not happy with the charging of the yellow top… it simply does not do the job quick enough… (driving one way for 500 mls or more and then a weekend will do, but not every day 100 to 250 mls every day every day (or every 2/3 days) i experienced
Now with a Redarc BCDC1225D and the same yellowtop, i have a lot more juice to spend and far quicker again… at every stop my yellowtop supports the snomaster at its full potential now..
(And a yellowtop is a deepcycle battery not a starter battery and for leisure and a serious fridge you simply “need” a deepcycle battery… those two do not do well together with spilt charge)
Just MHO…. From experience..Best regards
Harold
Always looking for Pre '55's & Pre war British Motorcycles! knowing or having one for sale? PM please. I visit the UK 6 times a year
Ps. I edit my texts quite often, english is not my native language, so I will edit My “typo’s” etc.
12th Mar 2022 8:32 pm
jenseneverest
Member Since: 12 Jun 2017
Location: somewhere
Posts: 765
well this install has not been as easy as first thought, space is really limited when it comes to actually doing it. I have the second batt mounted and linked to the main via 170amp VSR with 25mm cable and Anderson connector run through to the rear, i found an old dipstick really useful for threading the cable through the plastic bits on the floor into the rear.
The aux battery box i had to lob a bit off the side with a grinder to get the battery to fit,
the mount i made with angle iron and 6mm thread bar.
I added a plate for the breaker onto the main batt stay with some rivets, like said space is actually really limited. But it all went in and i can still use the battery covers.
Made a grommet from 20mm silicon water hose for the firewall, that is the last place i want the positive cable rubbing through..
the wooden box thing is the other 2 batts / invertor on wheels..... that goes in the boot when in use, only thing is it weighs about 70kg
3rd Apr 2022 12:03 pm
jenseneverest
Member Since: 12 Jun 2017
Location: somewhere
Posts: 765
Just to round off this thread.....
I finally got round to using this for a 4 day trip last week.
it had a constant draw of around 220watts, running 3 fridges 24 hours a day
it also boiled the 800w kettle each morning for the mandatory coffee's
I charged it twice a day for about an hour off the disco each morning and evening and it worked great.
The disco seems to peak at about 14.4 volts when charging, then settles quickly to about 12.8 volts when turned off, for my needs it worked perfectly, normally i would only run the one fridge and a few lights.
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