This applies to all equipment, just not trucks, but I asked here. for some reason, I seem to have better luck with just two big batteries, as three small ones, even though the CCa adds up around to the same # or even more with the small batteries. Just wondering what everyone else thinks. And is it just me or are batteries not getting any better. I used to figure 5-7 years, now I feel lucky in the 3-5 year.
In the off grid power industry, where people live off lead-acid batteries, we always say that less (larger) batteries in parallel cause less trouble. Each battery seems to have its own natural chemical voltage and if one is slightly different than the next, one will pull down the other or keep the other from fully charging and the end result is they both go bad. We have also noticed that off grid batteries from the plastic battery manufacturers (like truck batteries but bigger) seem to be declining in quality and longevity but they are not necessarily the same brands/mfrs as truck batteries. So I can't say for sure if it has any bearing on your problem. East Penn/Deka seems to be my preferred brand for small stuff at the moment.
As BV said, each battery of a parallel system has a natural voltage. The difference means one will be charging from the battery with higher voltage. Then the process reverses ... the batteries swapping voltage between each other. Eventually, the system, self discharges over time. Mismatched batteries like new & used, make the issue worse. If the batteries are used & charged frequently, this isn't much of an issue since they're being charged often. The other problem is when cranking, the load is most on the battery with the lowest resistance. Three vs two batteries just increases the odds for imbalance and failure. Batteries in series, avoids these issues. Plus, higher voltages like 24 or 48 volt systems can have smaller cables for the same power.
This was really clarified by International's 3+1 electrical system from the 90's. Plagued with starting problems. 1 battery to support running functions, 3 available for cranking only. Small trickle charge while running, but no drain while running, except that truck functions gobbled up majority of alternator output. So the "starting " batteries starved and died usually one failed and took the other 2 with it. Exactly as BV pointed out.
But, you kinda never see this problem in 24/12 systems . Corrosion is the biggest problem there.
Also, if we are talking trucks then it is probably 8Ds or 31s correct? A pair of 8Ds is a lot more battery than a 3 pack of 31s. Then again, it seems like 31s are a lot more popular and maybe manufactured more carefully these days. 8Ds are kind of dinosaurs. Plus how much fun it is moving 8Ds around, heh.
Yeah, with modern 12v trucks, never see anything but 29's and 31's . 24 v anything seems to be all 4D and 8D and of course they are never waist high. Always down deep in a well or counterweight. Or up in a box 2 foot deep on steel tracks. Only safe way to handle them is by crane, but there's almost always overhead obstruction. Loads of fun.
The biggest problem I've seen with multiple batteries is the number of connections. The positive terminals always seem to get the issue of corrosion. Each terminal has different resistance through it so more amps come from the battery with the least resistance. The big batteries also seem to stand up to vibration better over time and can stand a slow drain on power for a much longer amount of time. Leave a machine with two 4D batteries sit for a month and you have a chance of it starting. Do that with a set of group 31 and they will be dead almost every time. The other issue I've seen is the parked machine or truck having a continuous drain on the electrical system from all the new components installed. Remote monitoring systems and fire suppression systems go a long way toward draining and premature death of batteries these days.
One thing to remember with group 31's is battery weight it does very between different brands and spec's. I've found the heavier batteries{more lead} last longer. IMO Truck Shop
I prefer 4 group 31s. 900 cca East Penn/Deka seam to so well for me. Always have shut off switches or a solar maintainer.
On my logging machines with dual parallel batteries I put a disconnect on each battery before they come together at the starter. After starting I shut one disconnect off and then later in the day switch the other disconnect on and the on one off so the other battery is completely charged.. Having them disconnected from each other when out of service keeps them from killing each other. Cold weather is famous for having a weak battery die and pull down the good one discharging it and causing it to freeze and break. I am getting good life again after doing this.
I wonder if you could get the same result only disconnecting the batteries only at the end of the day. I say this because the alternator will provide a charging current until both batteries are within the charge voltage. As long as one of the batteries is low, the alternator will sense it and charge it. The other battery if it's fully charged won't accept any more power except as an equalizing charge. Disconnecting the batteries at the end of the day stops any parasitic draw & fire hazards too.
Never thought about doing it that way. Makes sense. On occasion we have a machine that'll have one bad battery suck down or flat kill the other good ones.
They only time I had trouble with one bad battery was when operators hooked up stereo systems and extra fans off one battery and didn't tell anyone. We would find them on Monday mornings or after a holiday weekend. Finally stopped it by confiscating the non conforming electrical gear and sending one idiot on to a career in another field.
Disconnecting the batteries would do the same thing. My heater fan which is the only electrical load while running is pulled from the starter hot terminal so is not a draw on just one battery.
The problem we have is every truck has an inverter so the highway chefs can run their micro wave, crock pot, toaster, blender, espresso machine, food processor, waffle iron, omelet cooker, the George Foreman Grill. So batteries get a workout. Oh I left out hair dryer. Truck Shop
All the years on the road the last place I wanted to fix a meal was in the truck I just spent all day driving. Damn sure didn't want to smell whatever it was I cooked all night. That's a small space to drive, sleep, lounge and cook in! Exceptions being the big bunks like my buddy's 150" but they have generators.