How much battery do I really need?

[Mirapei]

I'm bringing my old P19 back to life, and of course I have to replace the batteries. The dead ones are 73 Ah gel cells, but the replacement cost for two of them made my heart beat somewhat strongly. Can I get by with a single 100 Ah? or less?

[Dave Scobie]

It all depends on how much electricity you use (I know that seems a bit obvious).

When I lived abound my Montgomery 17 for 10 weeks I never once hooked up to 120v power.  Did fine with a Group 24 and a 40 Watt solar panel.  Used under 10amps a day: charge cellphone & GPS & laptop, and running cabin & anchor lights.

On my 25' on deck liveaboard had no issues going five days between charges, no solar so used a Honda8 outboard's 12 amp alternator for charging (only really putting out 8 amps or so as never needed to run motor close to max output RPMs. Boat had two Group 27 batteries.  Amp use a day was 10-12.  Used more power when sailing and running GPS, VHF and autopilot.

On my current liveaboard, a Baba30, have two Group31 batteries.  Easily do three or four days drawing down to 60% (not usually but happens on really cloudy periods). Only have a 50w solar panel that isn't well place to avoid shading. Just last week went three days and only drew down to 83% as had very good sun and boat cooperated and sat without much shading - generated 20amps a day from the panel.  Excluding the fridge (uses between 3amps and 7amps total a day, depending on outside temp and which side of boat faces the sun) boat draws on average one amp an hour ... so about 30 amps a day.  When sailing, with all electronics running, boat uses 5-6 amps and hour.

You need to evaluate the electronics you are using and how they are being used.  With this you create an energy budget and figure out the storage capacity be needed.

Lead based batteries (wet, AGM and gel) you don't want to run them below 50% charge, 12.1 volts, and then want them back up to full charge ASAP.  Lithium-Iron batteries can be drawn down to almost 10%.

[DBthal]

Dave,

Good results!  Are you using a MPPT controller with your solar panel?

Thx

[Dave Scobie]

MPPT controller with your solar panel?

Nope ... PWM only because what boat had when purchased so not replacing at this time.  Anyone putting together a new solar system should use MPPT (if bank account allows  )

[Scooter]

No idea if this is going to work out but my '85 P-19 wiring was so bad I ripped it all out and went with a handheld VHF and a dinghy light kit. Add a couple of camp lanterns and a bluetooth speaker and I'm thinking I'm good.

[pgandw]

I'm bringing my old P19 back to life, and of course I have to replace the batteries. The dead ones are 73 Ah gel cells, but the replacement cost for two of them made my heart beat somewhat strongly. Can I get by with a single 100 Ah? or less?

The use case is everything.  Are all your lights, including running lights, LEDs?  How much night sailing and/or anchoring out?  Any other electronics?  How many days between recharges?

My Mariner 19 came with running lights installed, but no electrical system whatsoever.  As I work to getting the boat fit to cruise for a couple of nights, the electrical system will be needed.  Besides powering lights (all will be LED), I need to keep my phone charged, which with the Navionics charts and app ($25/yr) serves as my navigation.  Maybe some day I will add a depth finder and VHF radio.  I plan a cheap 100AH LiFePO (if just under $400 is cheap!) which should be more than sufficient for my use case.  I will not bother with solar because of difficulty in finding a location where no shade line falls across it.  But the Mariner lives on a boat lift at the dock where there is 120V power to run the lift and power a charger when I arrive home (or tie up at another dock).

From the RV world, 2 GC-2 golf cart batteries are a lot cheaper and offer better power than 200AH of gel cels (AGM).  But flooded lead acid batteries can/will leak at extreme angles of heel, so I won't use them on my Mariner.  Our 2 GC-2 batteries (105AH usable) will run the camper for at least 4 nights, with the heater (3 amps running) being the primary load.  Other loads (LP fridge control board, CO/propane detector, occasional water pump and interior light use) adds up to about 12AH/day.  The figures have proven in real life.

Fred W
Stuart Mariner 19  #4133
Yeopim Creek, Albemarle Sound, NC