Battery System Design For Family Boat

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Fred
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Battery System Design For Family Boat

Post by Fred »

I've been thinking about battery systems for a few days now and want to record my thoughts for others to learn from or add to.

Boat hasn't been used much for a couple of years and this has resulted in 2 out of 3 batteries totally dead - none of them can put out enough current to start the engine or pull up the anchor.

Two brand new 260AH AGM batteries were installed a while back and dad had an old but still working gel battery in the stern that he could manually parallel up with the house load for some reason (goes direct to the main circuits).

Some history:

How It Originally Worked
  • Two batteries
  • One main switch: 1, 2, both, off
  • One main circuit for start, and house
  • A smaller switch that isolated all nav equipment from the main circuit
  • For part of that time we had a heavy duty diode bank that allowed lowest bank to charge first - that was discontinued at some point
That worked well for over 20 years with the following usage strategy:
  • Cruise on both
  • Isolate completely when moored with off
  • Isolate completely overnight while asleep with off
  • Switch to 1 or 2 before killing engine after anchoring (alternating between each bank day by day on a longer trip)
  • Chew through that one battery with lights, pumps, electronics, etc until bed time
  • Switch to 2 to start the engine
  • Switch to both once started
  • Pull anchor up with engine running and both batteries in parallel
  • Batteries recharged while cruising to next location
Not perfect, and a bit manual and laborious, but for 20 years that worked well and we were AFAIK never stranded without the ability to start the engine and pull up the anchor. There were a few cases where it was left on both overnight and the engine only just started, though.

Status quo: How It Is Now

Done on insurance a few years ago by an marine electrician:
  • Dedicated start battery - isolation switch cuts off starter and key and kill button somehow
  • Dedicated house battery - isolation switch cuts off house circuit, pumps and things
  • Override link switch to bridge the batteries together for whatever reason
  • VSR (voltage sensitive relay) that bridges the house bank to the start bank if the start bank is higher (I think, WTF if right)
  • Some kind of accessory feed system that keeps the electronic systems connected ALL THE TIME even when the HOUSE BANK IS ISOLATED (and resulted in a dead house bank within a few months of being installed)

This has not been working for us at all. It sucks. The saving grace has been the old gel battery in the aft compartment that is wired into the house circuit directly and that you take the terminal off to isolate.

To get the boat usable for a little holiday away from terrafirma I put together a 1300CCA/800A-for-30s/160A-to-empty 16 cell headway 38120HP pack and replaced the 260AH starting bank battery with it. I nearly got a hernia and nearly sank the boat with a battery shaped hole in the bottom, but with some screaming and help from wife, we got it up onto the deck leaving room for the baby headway pack. Observations:
  • 1300CCA is plenty to crank over and start the 8 litre C series Cummins (starts instantly) but in the event of some kind of issue, eg air in the fuel system, 32AH is not much capacity for repeated attempts
  • Alternator goes to too high a voltage (15.2 or so) when unloaded (bad AGM batteries and full/fresh headway pack) and this can/will damage the headway LiFePO4 pack
  • Isolation switch cuts off start bank from charging as well as loads, so is ALMOST fully isolated and can sit in fully charged state fairly healthy while cruising etc in this state
  • Bridge switch feeds charging into starting bank even if starting bank is "isolated" and must be off to avoid over charging the lithium pack
  • VSR can trigger without engine running and link good headway pack to bad house pack thus draining the headway pack for no good effect (ill intention of charging bad pack from good...) - I think this is why the starting wasn't as crisp after we left it all connected for a day and a half
  • Anchor winch will not work without the lithium bank in parallel - the bad banks cannot produce high currents even after hours charging and even with the engine running - they're knackered
  • Alternator current supply is less than winch current demand
  • With the engine running the 32AH headway pack can lower and raise the anchor a decent distance without running out of juice (but not a lot of margin in deep water)
  • After draining the headway pack the alternator has a load and the system voltage is determined by that pack - you can watch the voltage sit at 12.8 under load and about the same while charging and hit the isolate switch when you get to about 14.4 or so at which point the voltage spikes to 16V!
All in all, semi usable setup. Need to disconnect the VSR to avoid draining the headway pack though... yuck.

The above learnings have got me thinking about what I actually want to do once the boat is mine and I can start spending some money on it to make it right and reliable and trustworthy. Those requirements and initial design thoughts in the next post.
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Systems and Requirements

Post by Fred »

Thinking about the different loads/purposes/charging methods:

Critical systems:

Starter motor: 500A continuous, spike to double that initially - current spike limited, but capacity to be used more before success necessary for safety

Winch for anchor deployment/retrieval - has 150A breaker on it that does not trip, but might do under full strain - was not under strain during testing yesterday - chances are this is 200A+ depending on the model, I'll have to check - ability to lower anchor fully and lift anchor under load fully and have enough to start from the same bank afterwards would be ideal - can be pulled up by hand, but difficult at the best of times

Other systems:

Navigation equipment - various electronic and electrical loads such as depth sounder, radar, auto pilot, chart plotter, nav lights and stern light, etc - typically used only during passage, but drift alarms at anchor can be useful - moderate drain when all on/working, leaky drain when all off - capacity/runtime limited - important, especially while making a passage at night, but paper charts and marks and traditional nav can get you through in a pinch

House systems - various - including inverter, water pumps (fresh, salt, deck wash), lighting including overnight riding light, 3 way fridge, usb chargers, etc - important for comfort and hygiene but not safety critical

Charging methods:
  • alternator (apparently currently has an external regulator, though it's not very effective at keeping voltage stable without a load)
  • shore power, traditionally plugging in to bring up or keep up the batteries was useful however may not ever be needed again:
  • solar - a panel or two on the roof should be enough to maintain the batteries even with some light residual load left on by accident
Charging behaviour:

Alternator should be able to charge all battery banks as needed simply and reliably
Paralleling up high-capacity low-internal-resistance lithium batteries that are at disparate states of charge is dangerous and not a good thing to do
Solar charge controllers should take their quiescent current from the panel, not the battery, so no drain overnight or if the panel fails or whatever

Thoughts on all above

The headway 38120HP cells are great for minimising capacity and weight for a given current output - ie, automotive starting use. For marine starting use, your life depends on it, and for that reason you need more capacity as well. Therefore using other cells that have lower current to capacity ratios and enough capacity to comfortably provide the 1000A current makes more sense.

For house use, I'm not sure what capacity we need, but I suspect it's not very much:

Pumps might run for 2 hours per night max combined (salt rinsing dishes/hands, salt deck wash cleaning fish blood, fresh for dishes, cooking, showers, teeth, tec) and draw maybe 20a, so that's a mere 40AH, let's round it up to 50AH
Lights used to be halogen but are now mostly LED, typically a lot of these are run in various parts of the boat for the majority of the evening (front cabin, main saloon, galley, cockpit, bathroom, flood lights outside the boat etc) 50W halogen would be 4A each, but the LEDs should be under 1A each, so let's say if we ran the whole lot from 6pm to 3am partying hard and being careless it would use 30A all together and thus let's call that 300AH
Inverter, charging phones, laptops, power banks, etc, chances are most of these charge quickly and don't use much juice, but let's go worst case all flat 6 25W phones, 6 45W laptops from empty to full in 1 hour 70*6 420w, rounding up to 512W or 40A draw and 40AH consumed, plus maintenance charge at say 128w/10A total for the rest of the night being 100AH
Nav gear maybe 5A for 20 hours, is another 100AH
Fridge would be another 10A for 20 hours, 200AH - in the past rarely used, but would be nice to have a good setup there that could be used full time without fear

Capacity for starting doesn't matter as long as it can dump out the current required for a short period which LiFePO4 batteries seem to be good at doing right down to the line.

Capacity wise, then 50+300+150+100+200= 800AH absolute worst case.

Common cell sizes out of china are 200AH and 280AH - 3x280 exceeds our worst case, and 2x280 is pretty close to it. These cells can do 2C for 60 seconds and 1C to flat. Thus 4S2P could do 560A continuous for cranking and 1120A peak for initial cranking, and could crank from full to empty for a full hour if needed - clearly this is enough with a low voltage alarm on the circuit side of things and ability to totally isolate the batteries for long term storage.

Double that and we have two banks for redundancy and no need for a starting bank.

For flexibility and control I think the following is most versatile:

Main power bus *can* connect to all the things
Each battery has a switch that pulls it into this bus or isolates it fully and totally
Each load type has a switch that pulls it into this bus or isolates it completely
Additional small switches for isolating the batteries from their balance boards and voltage display screens
Current shunts sized to each component for each input/output for monitoring what's going on and where (sized as small as possible for precision of measurement on house bank or
Sell VSR unit
Sell bleed unit
Buy more simple heavy duty switches IF needed:

batt 1
batt 2
starter, key, and stop button
anchor winch
nav
house main
inverter

Alternator through FET-diode bank to each bank direct to charge lowest first - maybe capacitor on the alternator side of the FET-diodes to ensure no spikes on disconnect etc. Or some safety and switchable alternator too - maybe with spanner-to-switch level tech though.

Solar per bank, controller per bank, panel per bank - isolateable from batteries for storage.

Then with both banks connected you can play and drink and mess around all day with no chance of draining them and for longer duration partying at anchor you get a fair innings of fun before the buzzer goes off to tell you she's gotten too low to keep using...

I think I'd be happy with that, it's simple, obvious, no surprises, and could easily pull up the anchor from full depth under load and easily start the engine under any conditions, and likely never trigger the low alarm under anything by the most extreme conditions.


Thoughts?
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