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Batteries questions- mixing 6V and 12V

6K views 22 replies 13 participants last post by  BarryL 
#1 ·
Hi,

The batteries on my boat are dead and it's time to replace them. There are currently three group 27 Interstate batteries. They are wired in two banks. Batt 1 is bank 1 on my OFF 1 2 Both switch and batt 2 and 3 are connected in parallel and are switch 2 on the switch. The batteries are mounted in a small tray, about 1/2" high, and securely strapped down. The base of the tray is exactly 12 1/4" long and 7 1/8" wide. The height is not a concern as there is no box, just the tray the batteries sit in and the strap to hold them down. There is no more room to add additional batteries and I don't care to add more batts.

My electrical needs are pretty simple. The boat is on a mooring, no shore power. Recharging is by engine alternator. Electrical power is used for instruments (plotter, speed, depth, VHF), etc. I do have 12V refrigeration but that is only used on weekend or longer trips. We do occasionally spend a night or two at anchor, so cabin lights (mostly LED) are used for a few hours and laptops and phones get charged. I would never need battery power to last more than 2 days, as I would either be in a marina or sailing somewhere else.

Anyway, the simplest thing to do would be to buy 3 new group 27 deep cycle batteries, like from Trojan, Crown, etc. However, if I want more power, can i install 1 12V battery for the engine starting and 2 6V batteries for the house bank? Note that the batteries need to be charged by the engine alternator. Will the 12V engine battery and 2 6V batteries (wired in serial for 12V) be charged correctly from alternator?

Thanks for any info you have.

Barry
 
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#2 ·
Yes, as long as the two banks take the same charge voltages they can be charged together. Regular flooded lead you can check with a hydrometer is excellent value and most "generic" for charging requirements.

2x 6V is excellent value, many just go with Sam's Club or Costco golf cart deep cycle, but you may need to get a bigger tray.

Such large-capacity deep cycle batteries can be used for starting no problem, so personally I'd want to use the "starter" bank also as "reserve" for the house banks, which can in a pinch also be a backup for cranking.

So 4x the same 6V if you have room would give great capacity.

Do you rely only on the 1/2/both switch for isolating/combining the two banks?

An automated isolate/combine device would be better IMO, let you routinely pull down some of the reserve capacity from Starter when needed, but ensuring it always has enough to crank, isolating when it gets to a low-voltage cutoff, e.g. 11.8V.

And makes sure they're always getting charged together, including off solar if you add that later, without you needing to remember switching manually.
 
#3 ·
what ever the large bank of batteries you use just charging with the alternator will not bring the batteries back to full charge unless you run the engine for many hours. If you leave the boat on a mooring it is best to install some solar to bring the batteries to full charge. would also consider an ACR if you have a separate staring battery and a large house bank.
 
#4 ·
I would just replace with new and keep it the same lay out . We went the 6v route and I kind of regret it , problem was the 6vs had pretty much the same foot print as the 2 12vs and I had to add 3rd. 12v batt . to make the system complete . We did add a 100w solar panel , but we are away from shore power for 2 weeks at a time in the summer .
 
#5 ·
I reached a point 10 years ago where I got tired of worrying about how much ah I had to make the weekend or get charged.

It was at that point I went the route of 6 volt to double the ah in basically the same foot print. No matter how you look at it refrigeration will draw out 40-60 ah per day. You can only deplete the bank to 50-60% so 2 group 27 would be about 100 ah apiece or 210 ah with only 100 ah usable. That quite low for a multi day use .

6 volts are smaller footprints as a rule but taller which doesn't affect you. You quite possibly could get 4 6 volts and literally have480 ah ...240 usable and increase your power 130% and never worry about power again. Having a third group 27 would also increase your ah by 33%

We went the 6-6 volt route or 720 ah and don't have to run the engine for over 4+ days for charging. That fits into our sailing type as we go multiple days without using the engine. You don't have to go to that extreme, but I would recommend you increase you home bank.

We also have a dedicated starting battery

I agree with the ACR and getting them separated by charging at the same time.
 
#6 ·
Hi,

The batteries on my boat are dead and it's time to replace them. There are currently three group 27 Interstate batteries. They are wired in two banks. Batt 1 is bank 1 on my OFF 1 2 Both switch and batt 2 and 3 are connected in parallel and are switch 2 on the switch. The batteries are mounted in a small tray, about 1/2" high, and securely strapped down. The base of the tray is exactly 12 1/4" long and 7 1/8" wide. The height is not a concern as there is no box, just the tray the batteries sit in and the strap to hold them down. There is no more room to add additional batteries and I don't care to add more batts.

My electrical needs are pretty simple. The boat is on a mooring, no shore power. Recharging is by engine alternator. Electrical power is used for instruments (plotter, speed, depth, VHF), etc. I do have 12V refrigeration but that is only used on weekend or longer trips. We do occasionally spend a night or two at anchor, so cabin lights (mostly LED) are used for a few hours and laptops and phones get charged. I would never need battery power to last more than 2 days, as I would either be in a marina or sailing somewhere else.

Anyway, the simplest thing to do would be to buy 3 new group 27 deep cycle batteries, like from Trojan, Crown, etc. However, if I want more power, can i install 1 12V battery for the engine starting and 2 6V batteries for the house bank? Note that the batteries need to be charged by the engine alternator. Will the 12V engine battery and 2 6V batteries (wired in serial for 12V) be charged correctly from alternator?

Thanks for any info you have.

Barry
I reread you post and see you don't want to add any batteries. I would get two of the largest 6 volt and increase the ah and get a group 24 12 volt starting battery.
 
#7 ·
I've had good luck with gel cells. About 6 yrs ago I went from Prevailer type to AGM. My understanding is that these batteries can take a higher charge rate than liquid batteries and that they can hold a charge for a long time. Mine have been holding a charge over the winter on the hard.

Depending on your loads (I also have 12 V refrigeration and a serious autopilot) the higher charge rate can be important if you are not motoring a lot. You really want to keep your batteries up and getting below a 50% state of charge (as Chef2 implied) is not healthy, so topping up the battery bank is important.

Whatever you decide, it would be best to verify that your new batteries are fresh.
 
#8 ·
2x 6 volt, 2x 12 volt, that won't matter. One will need to be wired in series, the other in parallel, but the total amperage AT TWELVE VOLTS will be the same regardless of how you make them add up.

It really sounds like the OP is confused about volts and amps and watts and being misled by that.

Might as well keep it simple and just replace the batteries, then learn to give them proper maintenance to get the longest life out of them.
 
#11 · (Edited)
The OP is in Long Island (NY).

Excellent 6V golf-cart batteries can be had for about $100 each. I have 6 Crown GC235 red-tops aboard and after 3 years service love them. IMHO, better than Trojan T-105's, made in the U.S., and considerably less expensive.

I much prefer 6V golf-cart batteries over 12V varieties because they are true deep-cycle batteries (most 12VDC batteries are NOT). Crown golf-cart batteries have the thickest plates in the industry, and are IMHO top quality.

Also, a minor point: putting 2 6VDC batteries in series requires fewer connections than paralleling 2 12VDC batteries!


Bill
 
#12 ·
Hi Guys,

Thanks for the comments and please keep them coming.

I *think* i understand the difference between volts, amps, parallel, serial, etc. The issue for me is to get the most power for the limited space I have. It seems like I can get much more power with 2 6V batteries than with 2 12V batteries. The highest capacity 12V I can find that fits my space is is a group 27, like the Trojan 27TMX. That batt has 1.4 KW, at a 25 amp draw the capacity is 175 minutes. With 2 of those I can double the numbers (350 hours at 25 amps). With a 6V I can fit the T145 Plus. That batt has 1.7 KW, at a 25 amp draw the capacity is 530 minutes. So way more power with the 6V batts

The other problem I am having is finding a place to buy these monsters. Can't ship them Fedex or UPS. Heavy and expensive for freight. There is no Trojan dealer near me. I can't find a place that sells Crown. There are a few places that carry Interstate.

I'm trying to get this settled now so that when spring gets here I will be ready.

And yes, I live on Long Island, so if you know of a local place I would love to hear it.

Thanks!
Barry
 
#18 ·
......That batt has 1.4 KW, at a 25 amp draw the capacity is 175 minutes. With 2 of those I can double the numbers (350 hours at 25 amps). With a 6V I can fit the T145 Plus. That batt has 1.7 KW, at a 25 amp draw the capacity is 530 minutes.......
This caught my eye, because I thought industry standard capacity ratings were done at a 20amp draw. If those are correct, theoretically the batts would have more than a linear increase in capacity at the lower 20 amp draw. Ironically, the dishonest usually lower the amp/hr draw to artificially inflate the total capacity.

I also wonder if those minutes are to fully dead or to only 50% discharged, which is the lowest a LA bank should be discharged. Finally, even if done correctly, the ratings are always for batteries that are brand new and properly broken in. They lose capacity at time goes by, let alone improper charging.
 
#17 · (Edited)
The batteries are mounted in a small tray, about 1/2" high, and securely strapped down.
If you plan to stick with flooded batteries be sure the compartment the trays reside in can serve as containment for battery acid eg: no drain or weep holes, no metal screws that could come in contact with spilled acid etc.. If you move to AGM batteries a "battery tray" is fine as electrolyte can't physically spill.

Also, with flooded batteries, make sure your installed battery orientation is correct:

Installation & Orientation of Flooded Batteries

The base of the tray is exactly 12 1/4" long and 7 1/8" wide. The height is not a concern as there is no box, just the tray the batteries sit in and the strap to hold them down. There is no more room to add additional batteries and I don't care to add more batts.
If this is indeed the inside measurement of the battery tray, it should fit most G-27 AGM batteries, you do not have room for a G-31. That said those trays would also fit GC-2 golf car batteries, as you said height is not an issue. Course if going flooded be aware of acid containment.

My electrical needs are pretty simple. The boat is on a mooring, no shore power. Recharging is by engine alternator. Electrical power is used for instruments (plotter, speed, depth, VHF), etc. I do have 12V refrigeration but that is only used on weekend or longer trips. We do occasionally spend a night or two at anchor, so cabin lights (mostly LED) are used for a few hours and laptops and phones get charged. I would never need battery power to last more than 2 days, as I would either be in a marina or sailing somewhere else.
I would suggest investing in a small solar panel & controller to top up the batteries when the boat is sitting on the mooring. Unless you are motoring for 10+ hours your batteries are not getting recharged fully by the alternator. Even AGM batteries require approx 6+ hours to attain 100% SOC from 50% SOC and this is with pretty much ideal lab type conditions..

Anyway, the simplest thing to do would be to buy 3 new group 27 deep cycle batteries, like from Trojan, Crown, etc. However, if I want more power, can i install 1 12V battery for the engine starting and 2 6V batteries for the house bank? Note that the batteries need to be charged by the engine alternator. Will the 12V engine battery and 2 6V batteries (wired in serial for 12V) be charged correctly from alternator?
Two G-27 batteries from Trojan, Crown or US Battery would yield a total installed house bank capacity of about: Trojan SCS-200 = 230Ah, Crown 27DC = 230Ah, Deka DC27 = 180 Ah.

Two GC2 6V batteries in series would yield anywhere from 220Ah to over 260Ah depending upon the model chosen but they are a battery designed from the ground up for an abusive environment and yield better cycle life (longevity).

Two 6V batteries in series are a 12V battery. So long as charging voltages are the same or very similar to your 12V starter battery there are no issues. For example Trojan's prefer to be charged at 14.8V so you would be wise to charge them at 14.8V and uses a Trojan 12V as starter/reserve and two 6V as house.

If all you have is a stock alternator then your not doing any battery any favors if the voltages are not correctly matched. Everything is a compromise. I would start with a small "top-up" solar array first..

The problem with typical G-24, 27 & 31 "automotive cased" batteries is they are really not deep cycle batteries, other than the sticker claiming so, when compared to a Golf Car or sweeper/scrubber type battery..

What is a Deep Cycle Battery?

Seeing as an alternator is not going to get your batteries full, in the time frame most sailors run the engine, mooring use is by definition PSOC (Partial State of Charge) use. Unless you can get a solar array that can top them up in a couple of days your battery life will suffer..

In G-24, 27 & 31 Trojan, Crown and Deka all rate their "automotive cased" "deep cycle" batteries at half or less the lab cycle life to their Golf Car level batteries. While G-24, 27 & 31 batteries can and do work you are not getting the cycle life to Ah capacity to $$ ratio that you do out of truer deep cycle batteries.
 
#20 · (Edited)
Seeing as an alternator is not going to get your batteries full, in the time frame most sailors run the engine, mooring use is by definition PSOC (Partial State of Charge) use. Unless you can get a solar array that can top them up in a couple of days your battery life will suffer...
Floating this as a concept, recognizing maybe too complex or not cost-effective for the OP.

Assuming usually-abusive alt-only PSOC situation remains, this as an alternative to the (IMO sensible) recommendation to add solar.

Bank1: 2x 6v GC2 true deep-cycle in two of the trays, to be used as starter and reserve for house loads.

Bank2: Firefly Oasis, group 31, so requires a mod to or replacement of one of the trays.

All external charging sources direct to Bank2, absorption range can be 14.2-14.6, ideal is 14.4V. No float required, but 13.2 at the low end for longevity, now mfg says higher is OK, up to 13.5V

Key advantage to Firefly is the chemistry is **very happy** in the alt-only PSOC situation, which would be abusive with ANY other lead-based chemistry, sealed or flooded. See the other Firefly threads for the occasional discharge-full charge cycle required to recover full AH capacity after regular PSOC usage.

This bank2 should be used as primary House, will likely last many thousand cycles if cared for properly, making it very cost-effective overall, despite the high up-front investment ~$500.

Bank1 is used for starting, and could be connected in parallel to share House loads via a low-voltage cutout, to disconnect from non-essential loads at a relatively high setpoint, e.g. 11.8V or even 12. This usage will reduce its lifespan a bit, but will extend the lifespan of the more expensive Bank2. Also, it increases the odds (IMO very slightly) of both banks being too low to start the engine, so maybe add a PPS-X type personal power pack jump-starter if belt and suspenders security desired.

Or just use Bank2 as House on its own, still have the ability to occasionally switch loads to Bank1 manually but only as needed rather than routinely.

If both banks are automatically charged by the alt when motoring, then Bank1 should be selected to be happy within the ~14.4V charging voltage required by the Firefly.

The other key element here, and apparently an unconventional approach - others have expressed alarm over this idea - a DC-DC battery-to-battery charger keeps the charge input going into Bank1 **fed by Bank2 even after the engine is off**.

The goal would be to run the engine long enough to get past CV/absorption, to say 80-90% on the regular FLA Bank1, at which point the Firefly Bank2, with its much higher acceptance rate, will likely already be full or very close.

This no-alt batt2batt "top-up" stage is to get the GC batteries up to 100% SoC every cycle, perhaps lasting several hours more, but at very low amps current.

This will greatly extend Bank1's lifespan, even if it is sharing House loads as above, and even more so if used only as Starter, only occasionally as a separate Reserve.

Yes, there are some inefficiencies in the B2B charging process, but since we're talking about very little total power and given the Firefly's tolerance of PSOC conditions, I personally am guessing this would be a worthwhile trade-off.

Some DCDC chargers even include a MPPT solar controller, if OP wants to be ready for that option later.

Or, if a relatively high-current B2B charger is used, that could be the only charge source for Bank1, rather than combining circuits while the Alt is running, and this would allow the two banks to have two separate voltage levels, e.g. Trojan's preference for 14.8.
 
#22 ·
#23 ·
Hi Again,

I removed my old dead Interstate batteries. The date code on them is october 2010, so I am happy with how long they lasted. Today I bought new batteries - 1 Interstate 27DC from Costco for $80 (plus tax). That will be the engine start battery. From an Instate dealer I bought 2 GC2 ECL UTL 6 batts for $141 each. I have charged them all and will install on the boat this week. I have also ordered the solar plan from Defender.

Thanks for all the comments.

Barry
 
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