Boat battery bank and electrics upgrade

John Willis

After mid-Atlantic battery failure, John Willis decides it’s time to upgrade his battery bank and electrics

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Boat battery bank and electrics upgrade

Sailing to the Azores recently highlighted some electrical weaknesses on my yacht Pippin that had me desperately changing batteries around at sea – never a good idea.

I was eight days out, 180 miles west of Vigo, Spain, and wanted to tackle the battery issue before the forecast bad weather struck.

It seemed only one of two domestic 105Ah lead acid leisure batteries was working.

A boat with a white hull in port

Pippin is a much travelled Frances 34 Pilothouse. Credit: John Wills

The engine bank consisted of a 105Ah battery, wired separately, and up front lurked a fourth battery for the winch and bow thruster, which I decided to commandeer for the leisure bank.

Naturally, the squall hit just as I was carrying the 23kg battery back along the cabin to the battery compartment under the skipper’s wheelhouse pilot seat.

Predictably I achieved nothing except blood, sweat and tears so I switched off the fridge, ate my last steak reserved for a special moment, minimised radar use and burned diesel for the next eight days to recharge the batteries with the engine’s 80A alternator.

Switches for a boat battery bank

Master switches for the two boat battery banks. Credit: John Willis

My old 100W solar panel did its best to help whenever sunshine peeked through the clouds.

Electrical woes continued when my anchor winch succumbed to its numerous immersions, and the Raymarine autopilot failed just off Terceira Island’s harbour, forcing me to helm for almost the first time in 16 days.

Advantages of AGM

Back home in Guernsey my marine electrician sucked his teeth and chanted technical spells over my battery compartment, with its spaghetti junction of cables and wires, ancient bus bars, split diode and numerous add-ons – a common sight in many 25 year old boats.

He recommended immediate and expensive surgery.

A solar charge controller on a boat

PWM Solar Charge Controller was reused. Credit: John Willis

I’d made a summary of Pippin’s amp consumption and although my domestic bank’s capacity was rather lean for long periods at sea, it was adequate for ‘normal’ use, so we agreed a marginal capacity increase was fine.

He recommended Full River absorbed glass mat (AGM) deep cycle batteries all round.

AGM batteries have a fine fibreglass mat that absorbs the electrolyte, offering useful advantages over lead acid batteries.

These are:

  • Spill proof
  • Maintenance free
  • Can be mounted any way up
  • Low self-discharge
  • Longer lasting (if charged properly)
  • More resistant to shock and vibration
  • Lower internal resistance, high output
  • Faster charging
  • Better tolerance of deep discharge

Nothing is free, and AGMs are sensitive to overcharging, much more expensive and, in the case of the very robust Full River batteries, much heavier.

Indeed, the two new battery banks weighed 30kg more, so with her new engine, Pippin is carrying the equivalent of one comfortably upholstered adult in her belly, though she doesn’t seem to notice.

However, it is something to bear in mind for similar projects on smaller boats.

Extra weight

Retrofitting an old boat is never cheap, as I discovered replacing Pippin’s engine, but I see no point in a job not done properly, especially if heading far out to sea; so, I was happy to let the marine electrician loose inside the battery compartment knowing such things take time and time is money.

For the domestic bank, the two Full River DC115Ah 12V AGM batteries were installed on end, creating more space, and improving accessibility – though at 33kg each I won’t be lugging those around at sea!

A boat battery bank on a boat

Main battery compartment is under the wheelhouse pilot seat. Credit: John Willis

They were connected in parallel to form a single domestic bank, and an identical engine battery was installed at the back of the compartment, with all new marine grade cabling and associated bus bars and fuses throughout.

They fitted snugly and though battery boxes are not strictly necessary, I retained the one in the bows.

Continues below…

Two Merlin Blue Sea System battery master switches were fitted, one for each bank.

The domestic switch has two positions: 1. Domestic and 2. Engine + Domestic, allowing the engine to be started even if the engine battery is flat.

The domestic bank can also be boosted by the engine battery if necessary, and I can also easily add a third domestic battery later should I succumb again to the urge to roam far.

Boat battery bank monitoring

The existing Victron BV702 smart controller keeps a beady eye on the battery banks, although I confess to not understanding every message it produces… but I get the basics.

The solar panel was retired and a Solar Technology International 120W semi flexible solar panel installed, overseen by another brainy little box: the existing PWM VS2024N solar panel controller.

Two BEP 140A voltage sensitive relays (VSRs) replaced the split charge diode, one serving the main batteries, ensuring the engine battery is charged before switching over to the domestic bank.

A smart charger on a boat

Sterling 1230 smart charger keeps an eye on the charging regime. Credit: John Willis

The second serves the forward, bow thruster and anchor windlass battery.

For that we installed a Full River Full Throttle 750-35 64Ah AGM battery, which is smaller – and at 22kg, 1.3kg lighter – than the previous 105Ah lead acid battery, yet still capable of a beefy 900 marine cranking amps.

Water had entered the anchor locker in heavy weather to windward soaking the dodgy winch cabling, so that was replaced with marine grade tinned wire and waterproof connections.

Charging in port

Battery charging in the harbour is controlled by the existing Sterling 1230 smart charger because it has an AGM setting, ensuring the correct charging regime for the new batteries.

However, we discovered my shore power cable’s inboard end had a male fitting, which is illegal and potentially deadly.

I’m still alive to tell this tale as I luckily always plugged in to the boat before the dockside supply.

A replacement 240V AC Marinco plug/socket kit was quickly fitted.

Electric cabling on a boat

A pair of BEP voltage sensitive relays and much neater electrical cabling. Credit: John Willis

A Sterling 500W inverter off the domestic bank provides the diet for essential small items, such as a phone, handheld VHF, Garmin Inreach, torch, my little tranny and even the Torqeedo outboard battery.

Finally, remembering my angst outside Terceira, I bought a Raymarine ST2000+ tiller pilot to connect to Hercule the Hydrovane; yet another trick up his sleeve for he comes with a tiller and even the smallest tiller pilot can easily turn his small rudder.

I economised by installing it as a standalone and, if I were buying a small new yacht for offshore sailing, I’d seriously consider a Hydrovane and tiller pilot instead of a complicated electronic ram autopilot – it will probably be cheaper.

I’m a numpty about electrical technical matters, but it all seems to work well and I’d have no hesitation in setting sail again for distant horizons knowing it is so much better than before.

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