House Battery Recommendation?

[fredthorne]

I’ve wired my 26X (“Courageous”) with a pair of lead acid 12v batteries, a dual charger (that can charge different types of batteries) connected to my 30 amp shore power system—and it all works great. To power my fridge, Keurig machine and C-PAP device (also known in my household as “The Horrible Machine”), I’ve been considering a friend’s suggestion to add a LifePo4 (lithium phosphate) battery. Ideally I’d also like to install a switch between “shore” and “on board” power so I need not unplug and re-plug in my A/C powered devices.

My other option is to replace my house battery with a LifePo4-type battery, and use the cigarette-lighter style plug for my low wattage fridge and The Horrible Machine, and leave the Keurig behind. (though inconvenient, French press prepared coffee tastes much better anyway.)

Questions:

1 - Can anyone recommend a Lithium Phosphate battery or configuration that has worked well for them? To enable cruising to our many remote islands in the PNW, I want to power a low wattage fridge, The Horrible Machine and possibly the Keurig for up to 2-3 days. (I’ll be looking to add solar panels to this configuration to extend our cruising beyond 2-3 days.)

2 - Assuming LiFo charger compatibility, if I replace the house battery with a Lithium Phosphate, should I be able to just leave the fridge plugged in to the cigarette-lighter receptacle being powered by the Litgium Phosphate battery, which is in turn being charged by 30 amp shore power? Or must I unplug the fridge from 12v and re-plug it into a 110v outlet every time I’m in port?

[Be Free]

I’ve wired my 26X (“Courageous”) with a pair of lead acid 12v batteries, a dual charger (that can charge different types of batteries) connected to my 30 amp shore power system—and it all works great. To power my fridge, Keurig machine and C-PAP device (also known in my household as “The Horrible Machine”), I’ve been considering a friend’s suggestion to add a LifePo4 (lithium phosphate) battery. Ideally I’d also like to install a switch between “shore” and “on board” power so I need not unplug and re-plug in my A/C powered devices.

My other option is to replace my house battery with a LifePo4-type battery, and use the cigarette-lighter style plug for my low wattage fridge and The Horrible Machine, and leave the Keurig behind. (though inconvenient, French press prepared coffee tastes much better anyway.)

Questions:

1 - Can anyone recommend a Lithium Phosphate battery or configuration that has worked well for them? To enable cruising to our many remote islands in the PNW, I want to power a low wattage fridge, The Horrible Machine and possibly the Keurig for up to 2-3 days. (I’ll be looking to add solar panels to this configuration to extend our cruising beyond 2-3 days.)

2 - Assuming LiFo charger compatibility, if I replace the house battery with a Lithium Phosphate, should I be able to just leave the fridge plugged in to the cigarette-lighter receptacle being powered by the Litgium Phosphate battery, which is in turn being charged by 30 amp shore power? Or must I unplug the fridge from 12v and re-plug it into a 110v outlet every time I’m in port?

TLDR: Develop a realistic power budget. Put enough LiFePO4 batteries to support it with a safety factor. One size seldom fits all but the right size will fit you. Big inverters need big wires. Avoid appliances with resistance heating wherever possible. Make sure your charger is big enough to get you back out on the water in a reasonable amount of time. Embrace the concept: "You can't take it with you."

The price of lithium batteries have come down enough to where they are almost always the best choice. That was not true a relatively short time ago. Your friend is correct that, over the long term, lithium batteries will be a better choice for a house bank if you are going to put anything more than a light load on them.

I think you are saying that you are considering a switch that will switch the AC outlets in the boat from shore power to inverter power. That is not a trivial or inexpensive undertaking. When you switch between the two AC sources you are not just moving an extension cord, you are swapping out the electric company. There are switches that will do this properly, but they are not cheap. There are even some which will sense whether or not you have shore power and swap the appropriate wires automatically. They are even more expensive.

When I was researching the feasibility of doing this when I working on my own boat the most reasonable version I found was one which had the transfer switch as an integral part of the inverter. That was almost two years ago so you should probably check for yourself. Markets can change quickly.

Even if you are sure everything you have on the boat now would be happy with a modified sine wave inverter, seriously consider installing a true sine wave inverter. They cost more but you won't have to worry about whether or not a device you want to use in the future is going to work well with the power coming out of it.

1. LiFePO4 batteries prices and offerings are changing so quickly that specific recommendations are likely going to be outdated. Take any specific recommendations as ballpark suggestions if they are based on purchases more than a few months old.

Start by making a power budget of everything you are using now and everything you will reasonably be using in the near future.
Ex:
fridge 45W, duty cycle 50%, hours used 24.
45W/12V=3.75A
24h*50%=12h
12h*3.75A=45Ah.

C-PAP 50W, duty cycle 100%, hours used 8.
50W/12V=4.17A
8h*100%=8h
8h*4.17A=33.4Ah

45Ah+33.4Ah=78.4Ah. Conservatively you will pull 78.4Ah out of your house bank every day to support these two devices. You mentioned 1-2 days away from shore power so double it to 156.8Ah. I'll leave it to you to decide how much of a "fudge" or safety factor you want to add to that. I'll use 160Ah going forward just to get it back to a round number.

LiFePO4 can use 100% of its rated capacity with minimal damage. I'd recommend not using more than 80% of capacity if you want your batteries to last longer. That means you will need a 200Ah battery to keep these two devices running using the assumptions above.

If you are going to use actual deep cycle lead batteries you should not go below 50% depth of discharge to prevent damage so you would need at least 320Ah of true deep cycle lead batteries (that's north of 200 pounds of golf cart batteries). You can cycle them like lithium down to 20% in a pinch but you will wear them out significantly faster.

"Marine deep cycle" batteries are slightly better than standard batteries but are not deep cycle by any significant measure. You should try not to use more than 20% of these batteries so you will need 800Ah to get the job done. That's not remotely feasible.

Obviously these are just examples and don't take into account all of the devices you will want to use on the boat. They are, however, some of the more power hungry devices you could have aboard so they help to illustrate how quickly you will use up battery capacity at 12V (nominal). Electrical appliances on a boat are hard to support absent an external source of power. You could say it gives new life to the old adage, "You can't take it with you!"

The Keurig is not going to put a significant long term strain on the battery but it will hit it very hard for a short time. You will have to run it through an inverter so the math is a little different. I don't think it's a good idea to make coffee this way on a boat running off batteries but here's an approximation of the math.

A Keurig will need around 1400W while it is heating the water. I don't use them often but let's just say it takes 5 minutes to get the water in the tank up to temperature. Since it will be running on AC you will need an inverter. I'm not going to go through all the math but I'm going to say a 2000W inverter will draw around 180A wide open which will need something in the neighborhood of a pair of 2/0 cables along with an appropriate fuse or breaker.

For 1400W it will be closer to 130A for 5 minutes is 10.8Ah for the first cup of coffee and maybe half that for any cups made soon after. Personally, I'd stick to the French press.

2. This question is much easier. You can leave your fridge plugged into the DC outlet (cigarette lighter socket) and keep the batteries charged while on shore power. That is probably the most efficient way of running the fridge assuming it runs "natively" on 12VDC (mine does). The only caveat is that the charger has to be able to put more into the battery than the fridge (any anything else that running on DC) is taking out.

That brings us to charging. In addition to calculating what battery you need to support your loads you also need to be able to recharge the battery in a reasonable amount of time.

In the example above you are pulling out 160Ah over a two day trip and them heading back to the dock to plug into shore power. That 160Ah has to go back into the batteries somehow before you go out again. Assuming you want to go out the next day and that you are still on the boat while it's at the dock, you will need 160Ah to charge the batteries and another 48Ah (in round numbers) to run the boat overnight.

160Ah+48Ah=208Ah overnight
208Ah/8h=26A minimum charge rate. There are losses in the wiring and the batteries themselves that I've not accounted for. Round up.

Call it a 30A (minimum) charger running at least 8 hours to get your batteries back to 100% while you are still on the boat at the dock. You can get it down to a 20A charger if you move the C-PAP and the fridge to shore power and use all of the charger capacity for the battery.

Realistically, you probably want at least a 50A charger for a lithium bank that could support what you are describing.

[fredthorne]

Thanks so much, Bill. This is of enormous help to me. The power budget idea is terrific and I now see is a must. As is the adage “you can’t take it with you.”

So the Keurig is out except while on shore power. Using a French press is hardly a burden and I really do like the taste. My 12v-native fridge on eco mode consumes 45 watts on average, so rounding up I think I’m looking at just 4 amps for the fridge. I have a separate portable CPAP battery that I know will go two days of use on a single charge so maybe that’s the best for The Horrible Machine. I think you’ve got me on the right track.

My 20 amp dual battery charger gives out 10 amps per battery, and the only thing running between voyages (i.e., at home slip) would be the fridge—I suspect this means I net about 10 in minus 4 out of = 6 devoted to re-charging. So methinks I still recharge just at a slower rate, no? Or just turn off the darn fridge while in slip and re-cool it when underway. (But it’s sure nice to have a cool drink on hand when you come periodically to attend to a chore at home port!)

Thanks again!!!

Fred
S/V Courageous (Mac 26X)
Coupeville, WA

[Be Free]

Thanks so much, Bill. This is of enormous help to me. The power budget idea is terrific and I now see is a must. As is the adage “you can’t take it with you.”

So the Keurig is out except while on shore power. Using a French press is hardly a burden and I really do like the taste. My 12v-native fridge on eco mode consumes 45 watts on average, so rounding up I think I’m looking at just 4 amps for the fridge. I have a separate portable CPAP battery that I know will go two days of use on a single charge so maybe that’s the best for The Horrible Machine. I think you’ve got me on the right track.

My 20 amp dual battery charger gives out 10 amps per battery, and the only thing running between voyages (i.e., at home slip) would be the fridge—I suspect this means I net about 10 in minus 4 out of = 6 devoted to re-charging. So methinks I still recharge just at a slower rate, no? Or just turn off the darn fridge while in slip and re-cool it when underway. (But it’s sure nice to have a cool drink on hand when you come periodically to attend to a chore at home port!)

Thanks again!!!

Fred
S/V Courageous (Mac 26X)
Coupeville, WA

Fred,
Thanks for the kind words.

I just want to make sure that we are talking the same language.

Watts divided by voltage equals Amperes (Amps) in a DC circuit (we are not going to worry about AC circuits). The number of Amps is mostly important for knowing how big the wire needs to be (and to calculate Amp hours).

Amps times the number of hours the device is in use gives Amp hours. Amp hours is how much charge you will be taking out of or putting back into the battery.

Theoretically, if your charger can supply 10 Amps and it does so for 5 hours then you would have added 50 Amp hours to your battery's charge. It's a little more complicated in real life but it's close enough for a first approximation.

You are mostly correct in your shore power example. You would have approximately 6 Amps available to recharge the battery while the cooler is running but 10 amps when it is cycled off. I am assuming that your fridge cycles on and off and does not run all the time. In my example, I assumed it was on half of the time. That would mean that you would average 8 Amps available for charging.

Once your batteries are fully charged then (assuming lead batteries) you would have a fraction of an Amp going to each battery and up to 4 Amps going to the fridge as needed.

Keeping the fridge running when you are away is unlikely to cause a problem, but...

If your current batteries are not lithium, if your fridge is running off of the batteries, if you lose power for long enough, and if the fridge does not have a low voltage cut off (that's a lot of ifs) you might do irreparable damage to the batteries.

If the previous run-on scenario is true and it's possible to run the batteries dead from the fridge it might be a better idea to plug the fridge into shore power when you are going to be away for a while (if that is an option). Just don't leave anything perishable in the fridge. Just something to consider.