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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 09-09-2006
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Talking Mother of All Battery Banks?

Seen allot on the "parrallel" thread about batteries. So I figured I would avoid that one all to gether. People have a lot of pride in those battery figures!!! I prefer Guninness.

Who's got some practical experience with running one big battery bank in lieu of two banks? Seems I could use one big bank, and charge it at the same time.

or

I could break up into two banks. Use one, and charge the other.

Issues? Depth of discharge cycles? Complications with charging systems? Simplicity? Redundancy?
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Old 09-09-2006
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The biggest problem is making sure you have enough charger to recharge the batteries in a reasonable time. Typical charging current should be 20% for the rated capacity, i.e. 20A for 100 AH, 40A for 200 AH, etc.

Depth of discharge issues are the same with two banks or one. Discharge bank one to 50% then switch to bank two gives you the same discharge time as setting the switch to “both” and discharging the entire bank to 50%

The switch gives you something else to fail and will have no redundancy, If the switch breaks then you will have to hardwire your banks.

Battery redundancy will be the same for switch banks or paralleled banks (and this is where some have differed). If one battery in a paralleled bank goes bad, loss of capacity or shorted cell, the other batteries in parallel will handle the load.

Troubleshooting paralleled banks is a little more involved, but not especially so.

As you have been reading the other threads you probably know my opinion, paralleled banks are my preference.
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Old 09-09-2006
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I'm leaning that direction, as well (parallel). I have solid grasp of the general concepts. However, I don't want to over look any specific issues with respect to hardware and "realistic" use (aka practicality).

I am fortunate in that I am in the design phase for a dream ship, and hope to be abe to influence future touble shooting and maintenance by arranging the design in such a way that considers pitfalls like the ones you just mentioned. Most important to me is to identify the weak points of any system, and plan for access to common points of failure for troube shooting and allow for easier jury rigging.


Nothing is perfect and stuff will always break when you need it most so . . .
To summarize what you have said . . . Sounds like I need to anticipate the need of switch failure and to re-wire batts if I rely on a switch bank, but be prepared to trouble shoot parrallel batts if I go that direction.

OK - I think I got it.

If I need a large Ah capacity (say 170Ah/day) it looks like paralleling multiple 12v batts may be a suitable way to go. (recharge Amps also being considered - of course.) This would appear to avoid some switchology and be simpler.

Another question involves what you mentioned about charge capacity. 20% of the battery Ah rating? What happens if I'm charging and using simultaneously?? How do you see that affecting the 20% rule? It appears that I could potentially reduce the ideal 20% charge rating you talk about by interfereing with the charge process while using amps at the same time. What's the impact here? (This issue makes me consider using switch banks so as to isolate the batts as they are charging and not interfere with their recharge.)
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As long as you have more amperage produced then you are utilizing you will charge the batteries. Just remember you must replace at least as many amp hours as you remove each day, plus probably 20% fudge factor to overcome the inefficiencies of charging.

The 20% charge rate is a maximum recommended and not damage the batteries. You can, of course, use less current but it will take longer to charge the batteries. If your amperage budget is 10 amps, and your charging system provides 20 amps, this leaves 10 amps for charging. If your budget is 25 amps, then 5 amps will be coming from the batteries, and they will discharge in spite of the charger being on.

There is a 20/80 rule of thumb for charging the batteries. To get the batteries to 80% capacity will take 20% of the charging time, the remaining 20% will take the remaining 80% of the time. What this means practically is that if it takes your charger 1 hour to charge the batteries to the float voltage level it will take an additional 4 hours to get the remaining 20% charge. I suspect that this is where most batteries are damaged, most people do not charge the remaining 20%. Batteries need to be charged fully each time to preserve their capacity.

If you don’t have more charging amps then your energy budget (nominal equipment draw) then even with two banks you can’t put the current into the batteries faster then you are discharging. So even with two banks, you will discharge one bank faster then you can charge the other leaving you eventually with two discharged banks. The equation can get more complicated if you are using only solar where you charge only part of the day.

Just remember, amp-hours in > amp-hours out.
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Old 09-09-2006
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One thing to remember, the battery system should be planned from the battery all the way to the distribution panel. Especially if multiple charging sources (ie: solar, wind) are used. There's a very good diagram and description of one in either the summer or spring issue of Boatworks.

From my reading, I've seen where alternator output, with a 3 stage regulator, should equal 25% of battery capacity. I would assume that includes a fudge factor over the 20% figure.

Also, most with multiple batteries, have a dedicated start battery, then a house bank of 1 to 4 batteries. In conjunction with a battery combiner, instead of a switch, you have a charging system that will charge both banks, but isolate the start battery for the house load. That gives you one good battery at all times for engine starting.

The charging system I'm installing on my boat starts with a high-output alternator, to a 3 stage external regulator, to a combiner, then to the battery monitor, and then to the distribution panel. My wind generator will be tied in directly to the house bank.

As I said though, look at as a whole system, not just individual parts.

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Old 09-09-2006
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I'd like to ask a question. If the max charge rate should not exceed 20% of ah, so as to not damage the batteries, and I have 220ah in my paralleled house and 105 ah in my engine starting battery, will I damage my engine battery by using a 40amp charger?
Please advise.
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Old 09-09-2006
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No. You're good to go.

Bill
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Thanks Bill. Must be the shortest discussion ever!
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Yep. Figured you didn't need the long version. With a 40-amp charger, you can't do much harm :-))

B.
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DAVE,
. . . there you have it. Just came back from the Marina and wondered why my Solar panel on my current boat was "putting out" but my controller was weak.

Switched the charging system to a different battery. Same thing . . .


. . . . I left a damn light switch on all week, long!!
ooops

Batts are fine, just not where I wanted them.
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