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Old 11-04-2016, 06:35   #1
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Combining T105 & 8D Batteries?

Hi All, I think the answer is OMG NO! But yes is so alluring I have to ask...

I'd like to have additional Amp/hrs on my house bank which is currently comprised of 3 to 4 year old Trojan T105s to provide just over 400AH at 12V.

I've got 2 big (double sized to normal) 8D batteries, also 3 to 4 years old - one to start each Ford Lehman diesel. The engines start instantly every time. I think they are Exide, but don't know if 8D are start or deep cycle.

Can I move one of the engine batteries to the house bank without much 'real world' issues for battery life? These batteries are all older as stated, appear to work well, have been properly maintained, etc. Adding 2 'new' batteries to the house bank isn't practical unless I repurpose/eliminate an 8D start battery anyway, to use its physical location. BTW, these are all lead acid.

I could do the re-purpose right now just by how I set my existing battery selector switches, so its easy peazy.

Usage if you care: we cruise for 6 months spring to fall, 80% at anchor. Engine alternator charging & generator charging via Heart inverter/charger with 3 stage profile, though at anchor I seldom take it all the way to absorption as those last electrons are the hardest. I try hard to use the current batteries only to 50%, but there are days... At a marina every week to 10 days and the charger takes them to float for a day or two.

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Old 11-04-2016, 06:54   #2
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Re: Combining T105 & 8D Batteries?

OK here goes yes you can definitely combine the t105's and an 8D.
Now on to the 8D it's likely a starting battery and not a deep cycle one ( yes they make both) here is something to consider. On my dads boat he has the Mitsubishi Chryslers. He has a GRP 31 to start each one of them. Side note two group 31 starting batteries take up little more room than a single 8D. ( something to consider when the time comes.

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Old 11-04-2016, 07:27   #3
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Re: Combining T105 & 8D Batteries?

I'm sure people will chime in and say I'm going to die, but I combine my banks with the selector switch by setting it to ALL.
I do this so that I'm discharging the big bank to a lower percentage than if I had separated them.
I figure if I can manage fuel, why can't I manage a bank?
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Old 11-04-2016, 08:04   #4
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Re: Combining T105 & 8D Batteries?

Yes but maybe not the most desirable thing to do particularly if the big boys are not deep cycle. (As noted above.) I don't know which type of battery would be most likely to have it's life shortened by pairing up with the other but my guess would be the 8D. If that were the case it might be worth giving it a try. I would hate to shorten the life of the Trojans. Would it be possible to parallel one of the 8Ds during anchoring and do a separate charge once a week (+-) when on a dock? At the very least I wouldn't condition the two different battery types together.

Another thought for the future. Do you have a genset? I have one and I start main engines off the main (house) bank. 840 amps of golf cart batteries will SPIN the Lehmans even at 12.1 volts. I've never tried it lower. My back up battery is on the genset and it is completely separate from the main battery. It is an AGM and gets charged by the alt. on the generator or it's own battery charger when on dock power. When cruising I seldom go more than a few days without running the generator so it is always recently tested and topped off. If I mess up or lose a battery on the main bank I can reconfigure and charge up the main bank with the genset. Or, if in a hurry I can start the genset and jump the main engines from the gensets battery. I tried starting the 6 cyl Lehmens just using the gensets battery and it worked fine when OAT was 70 F. If it was 30F in the engine room I would definitely start the genset before trying to start a main engine using the little (group 31) generator battery.
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Old 11-04-2016, 21:37   #5
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Re: Combining T105 & 8D Batteries?

Can't see it hurting anything whilst ever they all remain relatively healthy. The least durable to discharge will have the shortest life.

However I had a mix of brands of deep cycle batteries all in parallel and everything was OK until one of them collapsed then it got as hot as hell when the others started to discharge through it.

It did not cause a problem because I noticed the low voltage on a large analogue voltage gauge and went looking for the reason but may have become serious had I been at the dock with a large battery charger switched on.
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Old 12-04-2016, 09:59   #6
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Re: Combining T105 & 8D Batteries?

My twin diesel engine powerboat came set-up years ago with two 8D batteries that were used for both house and start functions. Never really had an issue except that over the years I have added all sorts of 12v goodies - ie multiple GPS, Depth sounders, chart plotters, engine room lites, radar, 12v head, etc. About three years ago I decided to leave the 2 8D's as house batteries and add two Group 31 starting batteries to handle the starting duties for the twin Diesels (Detroit 8.2L ) and genset (8KW). There was no room to add another 8D. Had to place the two 31's outboard of the starboard engine and not possible to slide an 8D through the hatch into that position - but could fit the 31's.
All has worked well and the engines start right up with no sign of hesitation from going to the smaller batteries (in parallel).
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Old 12-04-2016, 13:06   #7
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Re: Combining T105 & 8D Batteries? has some FAQ that may address what you want to do.
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Old 12-04-2016, 15:38   #8
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Re: Combining T105 & 8D Batteries?

Thanks all. To answer a few of the questions:
, -yes I have a gen set with a dedicated start battery.
- yes I have a separate charger that I can use for just the start batteries when on shore power (and likely when on gen power, though I've never tried).
-the gen doesn't have an alternator, but has a 2amp pickup of the gen for charging it's start battery.
-the cable run to the 8d are about 6' of 0 cable to and from the rotary switch.
-I combine the banks when the engines are running to charge of both alternator.
-I'd only leave the one 8d combined while at anchor.

Thanks again.

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