Battery Banks and Charging

[SVPennyLane]

Quick newbie questions for the experts out there…
Regarding a cruising boat with 2 banks of batteries and a Diesel engine, starter, and alternator,
I am a bit confused as to how to calculate the amount or battery needed. From what I’m reading in a cruising book 2 banks are recommended to make sure if you drain on bank using usual systems for living (lights, pumps) then you have a backup to start the Diesel engine.
The book explained how to calculate the recommended amount of amp-hours needed for the House battery bank. Then it explained how to calculate the recommended amount of CCA needed for the Diesel Engine battery bank.
Or did I completely misunderstand and that the calculations for the House and the Calculations for the Diesel engine starter are combined for how much total recommended amp-hours, amps, and CCA needed. Then just create a whole other bank of the same thing as bank 2.

- Are both banks of batteries identical?
- Should the 2 banks of batteries have the same amount of amp-hours, CCA?
- The same amount of batteries?
- The exact same battery types?
Also,
Charging Batteries –Because I understand charging batteries is a bit of a risky situation because over charging can be damaging just like undercharging can cause un-repairable sulfation. My question is how do you avoid the issue of overcharging when for example, if the switch is set to “BOTH” and the engine is running hence charging both banks, however, Bank 1 is at 50% discharged and Bank 2 is at 5% discharged (or Full). I assume that Bank 2 would quickly become overcharged while Bank 1 takes a lot longer to charge to full.
And then what if for example the switch is set to “BOTH” and your using power with the typical house items, if Bank 1 is at 80 discharged (Empty) and Bank 2 is at 5% discharged (Full). Would both banks slowly drain causing bank 1 to be severely undercharged over time?

Any help would be greatly appreciated.

[msponer]

There are lots of ways to do this, but the most common is:

"Engine" bank is a single, normal, car-size battery.
"House" bank is all of your other batteries.

Both banks do not have to be identical.

They do not need the same number of amp hours. In fact, on my last boat the House bank was something like 1000 amp hours, and the Engine was about 100.

You do not need to add the CCA the diesel engine requires to the House bank.

It is usually very important that both banks of batteries are the same kind (wet, sealed, AGM). The brand or capacity is not important. You can get around this with fancy non-normal charging equipment, but there's really no reason to.

For my measly 75hp engine, a "normal car battery" was good enough, and there was no need for paying attention to CCA. If you want to do math, I'm sure it will work out that a motorcycle or lantern battery is too small.

It's completely normal to connect both banks of batteries together and charge them together. Many boats have a "1 - 2 - BOTH - OFF" type switch that does this (where 'BOTH' connects the banks together), though there is a more modern way to wire the banks where you have just a relay and an "ON - OFF - Reserve" switch, which is considered slightly more 'stupid proof'.

It's OK to leave the battery switch on BOTH while you charge them, even if the banks have a different state of charge. As a physics nerd, there are many ways to explain this, but maybe the best level of abstraction is that lead-acid batteries eat current slower as they become full. On another level, they have higher resistance as they become more charged, so the battery that is less charged will eat more of the current than the more fully charged battery. You won't overcharge any set of batteries unless the charging system (alternator) freaks out (is set to the wrong voltage). This is the same thing as saying that it takes longer to charge a battery from 95% to 100% than it does from 50% to 55% -- it's eating slower. If it's on the same buffet table as it's obese and hungry friend (the house bank), then it'll get almost nothing until they are about equally full and bloated (maybe I am taking this too far).

[djmarchand]

Mponer's explanation is exactly correct and rather articulate particularly the eating part ;-). One thing he left out is how to use your two battery banks and how to set the 1,2,all switch.

At the dock, while connected to shorepower charging, set the switch to all so both banks get charged. When you are ready to go cruising start the engine. Some anal types would say to first switch to the starting engine battery, but starting with both battery banks connected won't hurt a thing. In any case switch to all after the engine starts.

Then power to your favorite anchorage, drop the hook and switch to the house battery bank. This way all of your house loads at anchorage will come from that bank and save your starting battery for starting.

Then when you are ready to go, switch to the start battery, start your engine and then switch to all to charge them both up from the alternator.

And as msponer said, all of the above can be avoided if the two banks are isolated with a diode isolater (not recommended), combiner, ACR, Echo Charger, etc which gives you a set and forget system.

Oh and the house battery bank should be deep cycle batteries. The cheapest and best IMO is a pair or pairs of 6V golf cart batteries wired in parallel/series.

[cburger]

[QUOTE=djmarchand;821396] In any case switch to all after the engine starts.

I was under the impression that switching between batteries with the motor running can damage the diodes in the alternator?

[cal40john]

cburger
Switching to off is bad. Having a switch that is break before make is bad, but none of the switches made for switching batteries is break before make, they are make before break so a battery is always connected.

msponer
I agree, but if you have a 3 stage smart regulator that can be set to a high voltage bulk stage then you can be applying a larger than desirable voltage to the engine start battery which has already completely charged while the house bank is still taking a fair amount of charge.

John

[NewMoon]

Originally Posted by cburger

I was under the impression that switching between batteries with the motor running can damage the diodes in the alternator?

Any reasonably modern 1-both-2-off switch will be a "make before break" design, which allows you to switch between 1 2 and both without breaking contact. Don't switch through OFF, however.


We like having one battery (or bank of batteries) for starting, and another bank for house. The starting batts are optimized for engine cranking amps - able to provide a lot of amps for a short time. The house bank is deep cycle batteries, able to provide a smaller number of amps for a long time. This setup provides a lot of funtion and longevity per pound of batteries.

If you don't have a combiner/ACR/VCR to make switching automatic, you might consider our approach to switching, which works well for us (a power cruiser with no generator):

When you need to start up, switch to the starting battery bank, After a few minutes (10 is probably plenty to recharge them if your start batteries and alternator are in good condition) switch to the house bank. Don't do any other switching until the next time you need to start up.

When we use this method, our house bank is usually well recharged in an hour or two of running, and we don't have to remember to switch from both back to house when we shut down the engine. If we happen to sit for multiple days without a charging source, long enough to run the house bank down more than usual, we still have a well charged start bank to get us going again.

We don't like to take the chance of running both banks of batteries down to an undesirable level, so the only time we switch to both is the rare situation when the start bank doesn't have enough juice to get the engine going again. Even in that case, as soon as the engine is started we switch to the start bank long enough to recharge it, and then to the house bank. No dead battery problems in 6,000 engine hours of cruising so far.

[savoir]

Your start battery system doesn't need to be a " bank " at all. Unless your boat is over 40 ft or so a single group 27 will do just fine. The size of your house bank will depend on how cold you like your beer.

[Stu Jackson]

[QUOTE=cburger;821435]

Originally Posted by djmarchand

In any case switch to all after the engine starts.

I was under the impression that switching between batteries with the motor running can damage the diodes in the alternator?

The trick is to remove the AO from the C post of the 1-2-B switch, run it to the house bank, use the switch as a "Use" switch, and avoid this possibility entirely.

The OP might be interested in this: Electrical Systems 101

[Kettlewell]

Stu Jackson's advice is good. In a nutshell I highly suggest redoing your system so that you have a large house bank of deep cycle batteries that gets all the charge from your alternator, solar panels, wind generator, etc. and a separate dedicated single starting battery that gets a trickle charge from your house batteries (through some device, I use the Trik-L-Charge) when there is excess charging current. The main reason for this is that batteries last longer if they are discharged less, so the bigger your main battery bank the better. And, always having a fresh dedicated starting battery means your engine will start in an emergency and your starter will work better and last longer. Set it up so there is some emergency way of starting from your house battery bank if for some reason you deplete your starter battery. But, if the starter battery is just used for starting, there is little reason it will ever be depleted so much it won't start the engine.

[GordMay]

Greetings and welcome aboard the CF, Pat.

[Rustic Charm]

Hi you all,
I've got a 35foot ketch which has a bank of four batteries for house and a sep start battery. I have a 1 - both - 2 (house) and off switch as is common.

It's completely normal to connect both banks of batteries together and charge them together. Many boats have a "1 - 2 - BOTH - OFF" type switch that does this (where 'BOTH' connects the banks together), though there is a more modern way to wire the banks where you have just a relay and an "ON - OFF - Reserve" switch, which is considered slightly more 'stupid proof'.

I've just pulled out the house batteries in order to treat rust under their rotton wooden battery box. Before I put it all back together I want to ensure I wire it up as best as possible. So, can someone explain this 'modern way' for me to consider please?
Ted TAsmania.

[Boracay]

I've wired Boracay so that the alternator charges the house batteries (2 x 6V deep cycle) with an Echo charger that charges the start battery once the house voltage gets high enough.

20W of solar has been added to keep the start battery fully charged.

The master switch is set to 1) lights/radios/TV/nav/etc and 2) windlass.

Many cruising boats would be set up to suit their owners. I don't think there is one right way.

[mikefossl]

Here's what I have. Just re-done in the last 6 months.

[Dockhead]

Hi: There is a ton of good information in the archives here -- have a browse through them.

I also recommend reading this terrific library of technical info on batteries and charging:

SmartGauge Electronics - Technical Section - page 1


Briefly, yes, you are thinking in generally the right direction:

1. Engine start should be separate from domestic battery banks.

2. No, they do not need to be the same size or type.

3. Ideally, each bank has its own dedicated alternator and battery charger. This is often unrealistic and so there are various ways to split the charge, some better than others. Probably separate AC driven battery chargers must be realistic in most cases -- highly recommended to do it that way -- or second best, a battery charger which has a separate output for trickle charging the engine start battery.

4. If a separate alternator is not realistic, then you will have to split the charge somehow. There are various good and bad ways to do this.

5. How you wire your batteries is very important. See: SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

Good luck!

[mitiempo]

I agree with Stu and Kettlewell and a few others.

And add:

I would use an Echo Charge to take care of the start battery automatically. This eliminates the problem mentioned by John of overcharging the start battery. The Echo will prevent this. It is a simple 3 wire install between the house bank and the start battery. Battery Chargers | Auxiliary Battery Charger | Xantrex

All charge sources should go to the house bank direct - not through the switch. This makes switching for use only as stated by Stu.

As far as batteries, as posted a group 27 for start is fine. For the house bank 6 volt golf cart batteries in series for a pair or series/parallel in multiple pairs are hard to beat - for price or longevity. True deep cycle batteries do not have a CCA rating - not being designed for starting it is not applicable.

A bank of deep cycle batteries will however have more CCA than a single start battery, it is just not a published rating.

The simplest is to wire the house bank to #1, and the start battery to #2 on the switch. When you arrive on the boat switch to #1 and use it for both house loads and engine starting. #2 becomes an emergency battery, kept charged by the Echo Charge for the time when it may be needed. When you leave switch to off.