Batteries, charge controllers..

[groundhog]

I appologise in advance for starting yet another topic of this sort.
I have been searching and reading previous posts on this topic, and my brain hurts.

I have a very simple set-up. Small boat, 27 foot. Two 12v batteries. Manual 1-2-both-off switch.

I killed one battery last year and want to buy a new one.
I am thinking AGM. I want to pull this battery out and sometimes use it as double duty for my dingy electric motor.

I also have a 5 watt solar panel.

I don't want to kill more batteries.

My questions..
-Switching to an AGM (group 31 is the largest that fits into my bilge area), will my automotive alternator (with built in regulator, i think) burn up?
-Can anyone suggest a charge controller that will fit my simple needs of handling the charge profile of the existing lead-acid battery and the new
AGM, keep my alternator happy, and switch in the solar panel?

thanks,
groundhog

[pschoonveld]

I'm in the same position; same size boat and probably same size batteries - Group 24's w/ ~75 amp-hours each. My research seems to point that a 5w panel is enough to keep the batteries up to level with standard discharge (1%/day), but not enough to make up for losses during usage. I'm been thinking about springing for a 30W panel (~$250) and a pair of low-cost controllers.

The only decent dual battery controller I've seen seems to cost $150+ from the lowest cost sellers and looks like more controller than necessary.

One controller is theoretically ok for the bank, but if you permanently wire them in, you need to add a switch to break the connection as the batteries would be wired in parallel. Lower cost controllers can be had online for $20-30.


Smaller solar panels don't seem like they're going to be sufficient to compensate for actual usage. Estimating somewhat conservatively, a 30W panel can put out around 10 amp-hours per day (dividing the wattage by 15 to get to amp-hours and then multiplying by an optimistic 5 hours/day of power) and a 5W only 1.65. Putting that into perspective, my Raymarine A65 at full brightness, an average VHF or stereo are all rated at around 1 amp per hour. Multiply by 5-6 hours of sailing and there isn't much left to charge the batteries for lights!

A random thing I found out is that if I moved to a Group 27 battery (2" wider than a Grp 24), the capacity goes up by 30-40%, depending on brand. I needed new batteries and swapped in wet-cell 24s this past weekend. I wish I had thought about this in the spring when I had more time and I would have put in a new battery box to accommodate the larger batteries.. I paid $90 for each and could have bought Group 27s for $100 each, both quality batteries from a reputable generator shop. The 27s are definitely worth the extra $10 if you can squeeze them in.

A great reference is Don Casey's article:
Installing a Solar Panel to Maintain Batteries by Don Casey

[groundhog]

Thanks.

I have been looking at various batteries and am under the impression that up to group 31, the width is pretty much constant. Width is my limiting factor in the space I have for my batteries so I will probably go with them.

I am surprised that nobody knows anything about my alternator question.
My fear is that if the AGM batteries have a much lower internal resistance, they will draw too much from the alternator, making it hot.

WHat is the limiting factor in what current a given alternator can put out?
rb

[tommays]

The real problem is how long you would need to run the motor to fully recharge the batterys

IT takes a LONG TIME to get that last 15%

[hellosailor]

"WHat is the limiting factor in what current a given alternator can put out?"
The carrying capacity of the output wire or fuse.

But in conventional installations you don't have to worry about that, you no doubt have an automobile type alternator with integral regulator and they are designed to work as a unit. You can't burn it out UNLESS you bypass or replace the output lead, output fuse, or fusible link, whatever protects it.

And in common use, if it has to put out too much it will overheat and throttle back on its own, unless you've got a real cheap old regulator design in there.

Group31 batteries fall within the realm of "automotive" and a single one won't be a problem. Two of them probably won't be a problem (in parallel) unless you've run them down too far. If you want some fun, run both down about 80%. Start the engine on one, then throw the switch to "both" (assuming you've got alternator protection so you can do that) and watch the engine shudder and the fan belt jump like a snake from the extra load on the alternator.

Even with group 24's.

Mixing one wet battery and one AGM is generally a bad idea, using two of the same kind is all you really need to worry about in your situation. The rest matters--but isn't critical. Plenty of web sites about batteries, charging, mixing chemistries, when your head stops hurting.

[DropTop]

pschoonveld: I'm looking at doing the exact same thing, also on a 27' with 2 batteries.

which controllers and what solar panel are you looking at installing?

[pschoonveld]

hellosailor:
a Group 31 is 95-125 amp hours. that's a big battery for a 27' boat. REALLY big. Not sure if I'd call that "automotive" unless you mean "big rig."

I've run down my 24s and my Atomic 4 with a 55 amp alternator has never had any issues with sudden voltage draws (like flopping the switch) or similar. Maybe it's just that mighty A4.

Droptop:
I put together a spreadsheet that breaks everything down. It is in a format for me, but maybe you could find it useful as well. PM me your email address and I can send it to you.

Overall, I've done a lot of shopping for controllers at Northern AZ Wind & Sun (http://store.solar-electric.com). The owner is really helpful and validated my two controller strategy. I will likely go for two small controllers from them.

The best deal on panels looks to be the BP 20 or 30w models available from affordable-solar.com. Controllers themselves, I can't seem to ascertain a significant value increase with expensive units versus $25 ones. My inclination is to skimp there as 2 cheap ones are the same price as one expensive one and I can't point to a particular feature difference between them at this small panel size.

As a side note, I also looked at Sailnet's store, where I have had great success with good prices. However, the basic information and product selection has sent me to more specialized places.

[groundhog]

I think if you pick the largest amp-hour batteries that you can fit will increase the life expectancy of the battery. Batteries stay alive longer if you draw them down smaller amounts. So that's the strategy behind picking larger vs smaller batteries.

These devices you are considering, will they know to switch from solar panel to alternator automatically, or do you have to throw switches and such?
gh

[mitiempo]

Hellosailor
I believe that if the max alternator output is determined by the wire or fuse capacity then they are too small for the installation. I would suggest in a proper installation the maximum output is determined by the acceptance rate of the batteries being charged. If lead/acid that would be approximately 25% of capacity of the battery. If you have for example a bank of two Trojan SCS225 batteries with 130 amp capacity x 2 = 260 amps total capacity. 25% of this is 65 amps and in bulk charge mode (less than 75% remaining capacity of batteries) that's the maximum they will accept. If the alternator is rated at less than 65 amps output with a 3 stage regulator it will max until either the batteries reach 75-80% level or the alternator gets hot and puts out less amperage. If the round trip of length is 10' and you calculate for 3% loss the wire should be #6 AWG. As long as the wire is #6 or larger it will not limit output and shouldn't be sized to do so. I would probably go a bit larger in my example and fuse accordingly. The fuse is sized for the wire not the expected load in the wire. The fuse should be larger than the expected current and smaller than the wires ampacity.
If the batteries are AGM the above numbers will change as they will accept a higher charge rate. According to this website (Deep Cycle Battery FAQ, or Absorbed Glass Mat Batteries) most AGM batteries have no charge or discharge limits, but I have also heard that it is approximately 50% of capacity. In the above example using the 50% measurement with the same capacity bank the acceptance is 130 amps in bulk charge mode. With 3% loss again and 10' return trip the calculator gives us #4 AWG. Again I would go larger so as not to limit the output of the alternator used and fuse accordingly.
As far as mixing AGM and lead/acid I would not suggest this. With multiple charging sources (alternator, solar, and possibly shorepower charger, although the original poster doesn't say if he has a shorepower charger) they will require their own controllers - eg regulator for alternator (either built-in or a separate 3 stage ideally) as well as a controller for the solar panel(s). To my knowledge they can be set for AGM or lead/acid batteries but not both at the same time for separate banks. Any shorepower charger I have seen is also set for AGM or lead/acid but not both. If you go with AGM they should all be AGM. Even if you charge house bank and use an Echocharge for start battery it will mirror the charging regimen that the house bank is getting.
Brian

[mitiempo]

Groundhog
The problem with using a very large bank is that the alternator (and shorepower charger if there is one) should also be increased in size. With lead/acid you should be able to charge at approximately 25% of capacity.
Brian