Battery volt drop from inverter.

[Pete7]

Peter, during this winters lockdown I planned to install a new house bank of 2 x 6v golf cart batteries. Even made some cardboard mock ups to check they would fit. First day of freedom and down the boat, only to find that they wouldn't fit Weeks of planning down the drain. Then I remembered Sterling doing some good offers for drop in LFP batteries so pulled the trigger.

I now have a hybrid bank with 120Ah LFP and 85Ah SLA. The 1kw toaster, 1kw kettle and hoover have been selected to run easily from a 1.6kw inverter. So slightly smaller kettle than say a 2 litre home version, but there is only two of on board so that's fine. I have seen 70A draw at night when there is no solar to support the demand from the house bank. This causes a voltage drop from 13.xx to 12.8 on the LFP battery. The kettle takes 4m12s to boil 1 litre. The LFP returns to 13.xx volts as soon as the kettle switches off.

I wasn't sure before installing LFP but now think its a game changer for small yachts enabling house hold items to be run off small house banks for the first time. An induction hob is next.

We may expand the bank slightly by adding a further 100Ah in due course but its not a priority so far.

Pete

[BAD ORCA]

Originally Posted by rslifkin

It's not necessarily a linear relationship between load and how badly the voltage drops. In general, I'd try to keep any load more than a brief peak to no more than 15 - 20% of the C20 amp-hour capacity for FLA, maybe 25% for AGM. There will still be a noticeable voltage dip at that load, but it should be manageable.

Just how much you can get away with will depend on voltage drop in the cables as well as how well various equipment you have running will tolerate a dip in voltage.

So if i had (5) 116 Ah Firefly's that number would be approx 145 amps?

116 Ah x 5 batteries= 580 Ah/4 (25%)=145 Amps?

[rslifkin]

Originally Posted by BAD ORCA

So if i had (5) 116 Ah Firefly's that number would be approx 145 amps?

116 Ah x 5 batteries= 580 Ah/4 (25%)=145 Amps?

As a ballpark, yes. I haven't dealt with a set of Fireflys in person, so I don't know if they may tolerate more before the voltage dip becomes excessive.

[BAD ORCA]

Originally Posted by rslifkin

As a ballpark, yes. I haven't dealt with a set of Fireflys in person, so I don't know if they may tolerate more before the voltage dip becomes excessive.

Thanks for the clarification. I only have a single Firefly at the moment so i dont either but am considering a 12volt water heater that would draw approx load similar to the op's kettle when on, sans inverter. i plan on (5) Firefly's for the house bank so am trying to also determine if its feasible and approx how many Ah's of battery bank i might need. Sounds like i might be ok.

[thomm225]

So you can go all out with battery power simply to get your 700 watt tea kettle to heat or use an old propane or butane hot plate to heat your water.

It's all up to you.

The 16 ounce propane tanks I use cost maybe $4.00 and last a few months for cooking and coffee in the am.

Propane hot plates go for around $40.00

Or you can build yourself a huge battery bank to do the same thing.

[Boatwright]

"For mid-sized cruising boats, say in the mid-30 foot size, daily loads range
from 100 to 150 ah. Cruising boats could be "real" cruisers who are truly
"out there" to weekenders who spend a weekend at anchor (not marina
hoppers) or a week or two cruising locally on vacation. These folks tend to
have house banks in the nominal 400 ah range, either with 3 or 4 12V 130
ah batteries or four 6V golf carts for 425-450 ah."

This is an excellent rule-of-thumb for many medium sized cruisers. We have 4 Trojan 6v 105's, 440 AH total capacity @ 12v. Our energy budget is 80 AH / day. We rarely see the batteries below 80%.

We have all LED lighting, Isotherm refrigeration in a very well insulated top-loading box, minimal electronics, propane cooking, with a tea kettle and a french-press for coffee. No hair dryers, electric coffee makers, etc.. We do have an inverter for convenience - a small blender for those essential frozen fruity rum drinks, some small power tools and a 300 watt vacuum cleaner which are used occasionally. With the vac running we draw about 30 amps which brings the fully charged battery voltage down to about 12.2 v for the five minutes we have it running and only uses about 3 AH.

200 watts of solar with an MPPT controller meets 100% of our needs in all but prolonged cloudy weather.

Another rule-of-thumb. Keep it simple. The reason most of go cruising is to enjoy the wind and sun, and the challenges of sailing and navigating. You don't need all the comforts of a suburban home to have a good time.

[Pete7]

Originally Posted by thomm225

So you can go all out with battery power simply to get your 700 watt tea kettle to heat or use an old propane or butane hot plate to heat your water.

It's all up to you. Or you can build yourself a huge battery bank to do the same thing.

Indeed, but you don't actually need batteries, a gallon of Kero for the stove and nav lights and away you go. If you want ice cream on a small boat, well then you need to looks to recent technology options like solar and lithium.

The sailing oceans guy, who has just done an Atlantic loop without an engine has just installed a pair of lithium batteries. Worth watching he recent YT video on it.

Pete

[Adelie]

Batteries with more plates would sag less but then they would be hybrid (Marine) batteries or starting batteries and would have less cycle life.

I think your batteries are fine, might want a larger bank.