Battery Size

[lehighsail]

I have tested my batteries and get mixed results as to whether they are still good so since they are new to me I figure I will just replace them to be safe. They are both Deka Marine and are part numbers DC27 and 24M5. My question is do I have to replace them with the same size and
specs or can I install new larger batteries? They are connected in series and both have
isolators (?) and there is a battery charger on board. Thanks

 

[Don S/V ILLusion]

If it really is an isolator, it creates an inherent 0.7 volt voltage drop which means your batteries will be constantly undercharged leading to premature death. Replace it with any of a number of combiners or relays widely available, one of which is commonly referred to as an ecocharger.

Batteries should be sized to provide sufficient power between recharge such that they never discharge below 50% rated capacity. Consequently, the answer to your q regarding size is dependent on your typical usage. Figure out how much you draw between charges and double that figure to determine what you need.

 

[Stu Jackson]

Don's right. Do or Google "boat energy budget."

Here's one:

The All-Important Energy Budget:

http://c34.org/bbs/index.php/topic,3976.0.html

 

[lehighsail]

I see what you are saying about use and budget. I will try to figure that out for future use. Right now, I do not know exactly how I will be using the batteries, anchoring or docking, so my question still is, can I go wrong by installing a too large capacity battery. To take it further, can I also add a third battery to the bank without doing harm?
Also, should I add a battery, connect it alone to Number 2 on the big red battery selector and use it only as an "engine starter" battery?

Thanks again to all.

 

[emergpa1]

I see what you are saying about use and budget. I will try to figure that out for future use. Right now, I do not know exactly how I will be using the batteries, anchoring or docking, so my question still is, can I go wrong by installing a too large capacity battery. To take it further, can I also add a third battery to the bank without doing harm?
Also, should I add a battery, connect it alone to Number 2 on the big red battery selector and use it only as an "engine starter" battery?

Thanks again to all.

Oh, what a can of worms thee does open!

 

[Bill Roosa]

Yes, as long as they are 12 volt, deep cycle and fit in the available space you will be fine. Bigger is always better with battery banks.
Adding a third battery to the banks just requires the battery and two cables to connect to the existing batteries. just make sure that they have the correct terminals to match your cables. Some have bolt on lugs only and some have "posts" that require a auto type connector.

 

[emergpa1]

Yes, as long as they are 12 volt, deep cycle and fit in the available space you will be fine. Bigger is always better with battery banks.
Adding a third battery to the banks just requires the battery and two cables to connect to the existing batteries. just make sure that they have the correct terminals to match your cables. Some have bolt on lugs only and some have "posts" that require a auto type connector.

Bigger is always better? We'll then how about 500lbs of always undercharged/over discharged flooded cell batteries poorly placed as to boat balance and access for maintenance and three of which are sitting at an angle so that part of the lead plates are always exposed?
I do see where you said "as long as they fit". I'm just wanting to point out some things to consider, not questioning your insight. Just to keep things interesting. A bit of dramatic theatrical conflict if you will.

 

[Don S/V ILLusion]

You can create the unintended consequence of constant undercharge by adding battery capacity without a corresponding increase in recharge capacity rate. The result being constant undercharge leading to premature death of your new batteries.
Stated differently, it's a system, not individual component decisions.

Obviously, a larg(ER)bank with the existing alternator or charger will take longer to recharge, sometimes substantially longer.

 

[Bill Coxe]

I would check very carefully as to whether they are connected in series. Idon'tknow of too many sailboats in the 32' range sporting 24 volt systems. FWIW.

 

[emergpa1]

Hell he's got a hunter, 500 lbs is about a fifth of his displacement. JK ( sorry I've been sitting in the boat in the rain all day and am bored) I'll understand ignoring me.

 

[Scott B]

I installed 4 group 31's deep cycle last year x 130 ah each and also have a group 27 start battery that I keep isolated on the battery switch. We anchor out 2 nights each weekend and do a couple of 1-2 week cruises per summer, all at anchor.

They all are under the starboard settee and actually balanced the boat nicely.

 

[mitiempo]

I agree with Bill Roosa - bigger is always better. This does assume space to install them properly.

If you double the size of a battery bank and waste power in one way or another they will take longer to charge. But for a given power usage the larger bank will be discharged less and last a lot longer. The cost of the extra battery or batteries will be more than offset by length of service they give you.

Your batteries are wired in parallel, not series. I am not sure what you mean by "..both have isolators..."

If it is a battery isolator with the alternator output wired to one lug with the 2 outputs going to each battery like pictured below definitely get rid of it. The best solution to effective charging is to use an ACR or Echo Charger between the banks and wire the alternator output directly to the house bank. This makes proper charging of all batteries automatic. Or alternatively use the 1/2/both switch manually to charge all batteries with the associated risks.

 

[Maine Sail]

I agree with Bill Roosa - bigger is always better. This does assume space to install them properly.

If you double the size of a battery bank and waste power in one way or another they will take longer to charge. But for a given power usage the larger bank will be discharged less and last a lot longer. The cost of the extra battery or batteries will be more than offset by length of service they give you.

Bingo, bingo, bingo we have two winners!!!!

Your batteries are wired in parallel, not series. I am not sure what you mean by "..both have isolators..."

If it is a battery isolator with the alternator output wired to one lug with the 2 outputs going to each battery like pictured below definitely get rid of it. The best solution to effective charging is to use an ACR or Echo Charger between the banks and wire the alternator output directly to the house bank. This makes proper charging of all batteries automatic. Or alternatively use the 1/2/both switch manually to charge all batteries with the associated risks.

You do know that diode isolaters were invented by battery companies, to kill your batteries early, so they can sell more lead...

Maybe not but they might as well have been... Dumbest invention for marine batteries ever....

I am currently in the process of replacing 10 AGM batteries that were MURDERED by a diode isolator, undersized wiring and a Hitachi alternator.. The poor owner got just two years out of a $2800.00 set of batteries. The folks who did the install simply dropped the AGM's in-place of flooded...... It was amazing he even got two years but this was only because his shore charger did not pass through the isolator and they got to 100% a few times per year....

Seriously a larger bank will last longer and be a significantly more efficient bank to charge vs. the smaller bank. This of course is within reason. A good rule of thumb is to size your "loads" then add another battery or another 100+ Ah's.. This will take you to shallower average discharges.

No one ever talks about the Peukert effect, because most don't get it, but it is a HUGE benefit to a larger bank....

Some things to consider:

* No bank will ever get to 100% via alternator unless you motor for 10+ hours

* A smaller alternator on a larger bank will remain in BULK longer. This makes it run at its most efficient state for LONGER.

* Charging a smaller bank more often vs. a larger bank for a little longer there is NO COMPARISON which method is more efficient. The larger bank charged less often and for slightly longer is the MOST efficient. We can't forget 20-30% charge inefficiency so when charring more often you lose more.

* The larger bank will be discharged less so cycle life will be longer.

* The larger bank makes capacity gains from Peukert effect thus you gain capacity beyond just the face value Ah rating.


Simply put a larger bank with smaller average loads will survive better than a smaller bank with the same "average load"...

Take a parallel bank of 4 X 100Ah batteries. You now have a 20 hour rating that can support a 20A load, or 5A per battery, X 4 = 20A. When you run this bank at an average load of say 8A you'll really have 503Ah bank NOT a 400Ah bank..

If you add just one more battery and make the bank 500Ah's and you'll have a 25A support load (at the 20 hour rate), BUT, apply the same 8A load and you have a bank that can deliver 665 Ah's using an average of an 8A load.

Conversely, size your bank small at 100Ah, which would have a 5A support, and still apply the same 8A load and you really only have an 89 Ah bank. Bank size vs. load matters and the bigger bank results in a lower Peukert corrected load. This means less capacity is used with a larger bank and the shallower the discharge cycle will be. Shallow discharges are good for the battery bank and deep discharges are bad.

If you keep your same charging schedule your alternator will still be in bulk longer, which is more efficient, and you still need to replace the SAME capacity you took out..... Instead of hitting "limiting voltage" at 75-80% SOC the larger bank with the same charge source may not hit absorption/limiting voltage until the 90% SOC range due to the current / Ah capacity ratio......


* Batteries should match if put into one bank. Match battery brand, battery model and date code!

* No diode isolators!

* Bigger is better, within reason!

* Charge them properly!

* Maintain them properly!

* Wire them properly!

 

[Bill Roosa]

another "battery buying concept" to consider.
if your house bank is "one big haunkin battery" it sees the full load so "C" for it is high
if you used 50 smaller individual batteries to make "one big haunkin batter bank" of the same capacity (all wired in parallel of course) then each one's "C" would be smaller and you would be only taking x/50 amps out of each individual battery for an load of x amps.
When you lower "C" for a battery you get more effective AH out of it because you are discharging at a lower amp level.
Sooo 4-6 volt trogins = xxx AH is not as "good" as 6 Costco 12 volters that add up to the same AH. Assumes you actually have the space to install of course. The smaller batteries are also lighter individually and can fit in smaller spaces.
FWIW

 

[mitiempo]

Sooo 4-6 volt trogins = xxx AH is not as "good" as 6 Costco 12 volters that add up to the same AH. Assumes you actually have the space to install of course. The smaller batteries are also lighter individually and can fit in smaller spaces.
FWIW

Not true because the 6 volt batteries (golf carts) have thicker plates and will be better able to handle multiple cycles. Costco 12 volt batteries are not "deep cycle" batteries - nowhere close.

 

[Bill Roosa]

Costco does sell deep cycle batteries and C (capacity/amps draw) has nothing to do with the plates. All batteries preform better when you draw smaller current from them.
So your 4-6 volt supplying a 10 amp current to the reefer see 5 amps each while 5 smaller batteries would only see 2 amps each.
if the total bank capacity was 400 AH then:
2-6 volters have 200 AH and 5 smaller would have 80 AH each
C for the example would be 200/2.5=80 and 80/2=40
so you are "working" the more numerous smaller batteries less hard than the "2" larger ones.
This is a commonly accepted fact that "working" batteries less hard results in more AH out of the battery relative to working it harder where there are less AH.

 

[Maine Sail]

another "battery buying concept" to consider.
if your house bank is "one big haunkin battery" it sees the full load so "C" for it is high
if you used 50 smaller individual batteries to make "one big haunkin batter bank" of the same capacity (all wired in parallel of course) then each one's "C" would be smaller and you would be only taking x/50 amps out of each individual battery for an load of x amps.
When you lower "C" for a battery you get more effective AH out of it because you are discharging at a lower amp level.
Sooo 4-6 volt trogins = xxx AH is not as "good" as 6 Costco 12 volters that add up to the same AH. Assumes you actually have the space to install of course. The smaller batteries are also lighter individually and can fit in smaller spaces.
FWIW

This is incorrect.....

4 - 6V series / parallel = 450Ah at the 20 hour rate

20 - 22.5Ah batteries in parallel = 450Ah at the 20 hour rate

If both banks have a Peukert of 1.25 then the Peukert corrected load is identical for both banks.

Each 22.5Ah battery can support a load of 1.125A at the 20 hour rate 20 of them wired in parallel can support a 22.5A load.

The 450Ah series parallel bank can support a load of 22.5A at the 20 hour rate.

Both of these banks will see the same Peukert correction if both banks have a Peukert of 1.25, which is a fairly typical flooded deep cycle Peukert exponent.

Where the 6V bank wins, hands down, is cycle life...... 6V batteries on average get DOUBLE the lab rated cycles when compared to group 24, 27 or 31 12V flooded batteries...

We can't just measure Ah capacity, rated cycle life is the main determining factor of longevity and a true deep cycle battery, like a 6V GC2, will have more cycle life. 12V G-24, 27, 31 and 4D and 8D are NOT true deep cycle batteries but GC2 6V, L16 etc. are true deep cycle batteries.......

 

[Maine Sail]

Costco does sell deep cycle batteries and C (capacity/amps draw) has nothing to do with the plates. All batteries preform better when you draw smaller current from them.

All batteries in the US are rated at a 20 hour rating for deep cycle use. A "larger" battery can support a larger load than a smaller battery will thus Ah to Ah's, when both banks are equal in size, at the 20 hour rate, the "load" the battery sees is the same in terms of the 20 hour rate.

A 225Ah battery can support a 22.5A load and deliver the rated Ah capacity

An 80Ah battery can only support a 4A load and deliver the rated capacity

See, NOT equal....

So your 4-6 volt supplying a 10 amp current to the reefer see 5 amps each while 5 smaller batteries would only see 2 amps each.

Bill, as you like to say "put your EE hat back on".... Let's run the math out...

4 X 225Ah 6V = 450 Ah The bank can support a 22.5A load and deliver the 20 hour capacity. Each 6V battery can support an 11.25A load when in series for 12V.

5 X 12V 90Ah = 450Ah bank (group 27's) in parallel can support a 22.5A load (as a bank) and deliver the 20 hour capacity. Each 90Ah battery can only support a 4.5A load not an 11.25A load to deliver the rated capacity..

so you are "working" the more numerous smaller batteries less hard than the "2" larger ones.

No you are not because the smaller batteries can NOT deliver the same "rated" load as the larger battery can. Ah/Ah at the 20 hour rate they will both deliver the SAME capacity and be "worked" identically.

Let's back into this with an average boat systems load of 5A..

A 200Ah battery can support 10A load so the 5A systems load is half what the battery is rated at. This battery will coast along with an easy life and deliver 238 Ah's, at a 5A load, when Peukert corrected with a 1.25 exponent..

A 100Ah battery can support a 5A load, and 5A is 100% of what the battery is rated at. This battery is running at "rated" load and will deliver 100Ah's, the rated capacity.

A 50Ah battery can support a 2.5A load and a 5A load is 200% over what the battery is rated at. This battery is working harder than it is rated for and will only deliver 42Ah's at a 5A load.

Take 4 X 50Ah batteries in parallel and that 200Ah "bank" can now support a 10A load, just like the single 200Ah battery. The "load" is the same on the 200Ah battery or 4 X 50Ah and is 50% of what the bank was rated at.... No difference and both banks will deliver the same Ah's..

Smaller batteries can't deliver the same 20 hour rated loads as big batteries, you simply forgot that point....

This is a commonly accepted fact that "working" batteries less hard results in more AH out of the battery relative to working it harder where there are less AH.

Working batteries less hard does extend life. However with Ah capacity being equal more smaller batteries DO NOT mean the bank is worked less hard because the smaller batteries deliver less "load" on an individual basis than do larger ones.

 

[Scott B]

Unfortunately, for the OP, as myself with the same boat, do not have enough height clearance to put Golf cart batteries in the current space where the batteries and charger are located. I ended up going with the 4 Crown Battery group 31's as they had good ah rating and were the same height as the 27's I had there originally.

 

[Benny17441]

Batteries rated in CCA or Cold Crancking Amps are usually starting batteries and not suitable for House use. Make sure you get Deep Cycle batteries. A lot of sailors employ Deep Cycle batteries for starting as the small auxiliary diesels start well with them and can be incorporated into the House bank if needed.