How many batteries

[Nickk]

Looking at replacing 3 x 10 year old Rolls AGM S12-128 batteries, I had a look at their cycle life vs depth of discharge (DOD) specifications . They quote 3500 cycles at 20% DOD, 2300 cycles at 30% and 1200 cycles at 60%. Mileage will vary of course but it was the relative figures I was interested in.

3 batteries at 20% DOD will give me 3 x 20% x 115AH = 69Ah. So will 2 batteries at 30% or 1 battery at 60%.

3 batteries at, say, £270 will cost £810. If that gave me 3,700 cycles it would cost £0.219 per cycle. Similarly 2 batteries, £540, 2300 cycles £0.235 per cycle and 1 battery £0.225 per cycle. Nothing in it.

Fewer batteries saves weight and keeps money in my pocket; more batteries gives a greater reserve.

I cruise in Greece, have 150W of solar panels and don't find it difficult limiting use to 69Ah. Should I drop down to 2 batteries or am I missing something?

 

[Habebty]

i’ve just fitted 3 x 100ah AGMs for £450 from 12v Planet. I wouldn’t worry about the weight which is a small part of the overall displacement.

 

[geem]

Looking at replacing 3 x 10 year old Rolls AGM S12-128 batteries, I had a look at their cycle life vs depth of discharge (DOD) specifications . They quote 3500 cycles at 20% DOD, 2300 cycles at 30% and 1200 cycles at 60%. Mileage will vary of course but it was the relative figures I was interested in.

3 batteries at 20% DOD will give me 3 x 20% x 115AH = 69Ah. So will 2 batteries at 30% or 1 battery at 60%.

3 batteries at, say, £270 will cost £810. If that gave me 3,700 cycles it would cost £0.219 per cycle. Similarly 2 batteries, £540, 2300 cycles £0.235 per cycle and 1 battery £0.225 per cycle. Nothing in it.

Fewer batteries saves weight and keeps money in my pocket; more batteries gives a greater reserve.

I cruise in Greece, have 150W of solar panels and don't find it difficult limiting use to 69Ah. Should I drop down to 2 batteries or am I missing something?

How long have the batteries been on the boat in a hot climate? Heat kills batteries. Cycles are normally quoted at 25degC battery temperature. You won't achieve design life cycles if the batteries are kept hotter than 25degC. If you keep your batteries at 35degC you half their life. If you are going to keep batteries warmer than 25DegC then little point in running more batteries than necessary. The perfect balance would be to hit the point where design life cycles and heat reduced life cross on a graph. For this reason we halved our 1000Ah capacity. We are likely to get the same life out of half the capacity. We are in the Caribbean

 

[Stemar]

I'm far from doing it - it seems my leisure batteries are doing their job, ably assisted by a fair bit of solar, but were I to be looking at spending real money on a battery bank, I'd spend a few hours wandering around YouTube and elsewhere finding our about LiFePO. The lighter weight and and greater depth of cycle before damage are both clear advantages, and it does seem that prices are getting to the point where it's starting to make sense even for those whose needs are less than extreme. Yes, you'll end up redoing your charging systems, but the amount of amp hours you can pack into the same space and weight does seem to be getting persuasive for anyone with serious off-grid ambitions.

 

[Neeves]

i’ve just fitted 3 x 100ah AGMs for £450 from 12v Planet. I wouldn’t worry about the weight which is a small part of the overall displacement.

Bigger batteries are much more difficult to manhandle. We have 2 x 200a/hr batteries and changing them is a major logistical exercise.

I'd have to disagree with the idea 'I wouldn't worry about weight'. The less weight you carry the more efficient your yacht will be in terms of its sailing ability. This may not matter to some or many - but if you don't worry about weight - you will be slower. But it is a philosophy -some pay attention some simply enjoy what they have.

I've just changed an anchor rode on a French circumnavigating yacht from 8mm to 6mm - to save weight (they deploy/retrieve by hand). They replaced their lead acids with LiFePO. Their mechnical power is an O/B and WattnSea I'm changing an anchor rode on a circumnavigating UK yacht from 10mm to 8mm to save weight and space, they retired their windlass (the shaft had seized) and replaced with a new, smaller Maxwell windlass. A Ovni replaced their 10mm chain for 8mm - to save weight. A new 50' cat rejected use of 10mm chain, for 8mm - to save weight. Weight is important to some

As Stemar points out LiFePO gives opportunity to totally re-consider weight, along with the rode. All the down sides are manageable and costs can offer savings (ands more so if you are considering a new yacht). It seems a no brainer now to look very seriously at LiFePO batteries and management and a lighter rode - if you are commissioning a new yacht.

Jonathan

 

[Buck Turgidson]

2 sounds fine to me.
Ignore the lithium option. $$$$$ for no return. Weight savings is a none issue on a 10 ton boat. Just be sure of your overnight max load. Think about an overnight sail with everything on from dusk till dawn. It's no problem for me as I have a continuous load of between 1.5 to 2.5 amps depending on the sea state (Autopilot load) and 218ah battery. But get two rather than one big one as moving mine is almost impossible singlehanded at 60kg.

 

[HissyFit]

Hand-in-hand with depth of discharge comes the consideration of the time that your battery will spend at a partial state of charge. No lead-acid battery likes being sat at a partial state of charge for long. It will reduce the maximum capacity of your cells faster than any other consideration.

With a greater capacity battery bank comes a greater charge acceptance rate. A greater charge acceptance rate leads to more efficient (and thus cheaper) charging, with less run time and wear-and-tear on your engine. It also reduces the time your batteries will be sat at a partial state of charge, even when charging off marina hook-up.

 

[jwfrary]

Hand-in-hand with depth of discharge comes the consideration of the time that your battery will spend at a partial state of charge. No lead-acid battery likes being sat at a partial state of charge for long. It will reduce the maximum capacity of your cells faster than any other consideration.

With a greater capacity battery bank comes a greater charge acceptance rate. A greater charge acceptance rate leads to more efficient (and thus cheaper) charging, with less run time and wear-and-tear on your engine. It also reduces the time your batteries will be sat at a partial state of charge, even when charging off marina hook-up.

Yes and no.

Yes bulk charge rate of the whole bank is a higher amperage, but there's also more ah to put back in!

Absorbtion takes that same time, and if you don't do that then you shorten life span.... Alternators without smart controllers don't do this of course anyway.

Lesuire/Marine use abuses batteries, by cheap, AGM start stop 100ah car batteries are quite good value and ubiquitous and use them they will fail at some point and you will have to change them its just a fact of life they are a consumable!

 

[vyv_cox]

I had 2 X 115 Ah battery domestic bank with about the same solar output. After running the fridge all night voltage was down to 12.0. I added another battery and morning voltage went to 12.4 V. A far more healthy situation.

 

[roaringgirl]

When my 540Ah (2 x 70kg) of AGM died due to overcharge from incorrect alternator installation and a lot of partial discharge (we live off solar, wind and hydro), we got 400Ah of LiFePO4 (2 x 20kg). The old batteries were a nightmare to lift out, the new ones were trivial. The old ones had a useable capacity of 270Ah, the new ones are 380Ah. The old ones had a dedicated battery box that took up a large portion of the saloon and limited the seating configuration choices, the new ones fit in a small box under a seat in the corner and allow us to have the horseshoe configuration we want. It is only in retrospect that I realise how much I used to worry about state of charge, and how little I worry about it now. We will see if the longevity of the LiFePO4 lives up to expectations, but the AGMs lasted 18 months and were half the price...

 

[jwfrary]

What

When my 540Ah (2 x 70kg) of AGM died due to overcharge from incorrect alternator installation and a lot of partial discharge (we live off solar, wind and hydro), we got 400Ah of LiFePO4 (2 x 20kg). The old batteries were a nightmare to lift out, the new ones were trivial. The old ones had a useable capacity of 270Ah, the new ones are 380Ah. The old ones had a dedicated battery box that took up a large portion of the saloon and limited the seating configuration choices, the new ones fit in a small box under a seat in the corner and allow us to have the horseshoe configuration we want. It is only in retrospect that I realise how much I used to worry about state of charge, and how little I worry about it now. We will see if the longevity of the LiFePO4 lives up to expectations, but the AGMs lasted 18 months and were half the price...

I'm curious, What brand of Lithium did you go for and how did you solve your dc alternator charging regulation, dc to dc charger or regulator controller?

 

[roaringgirl]

What
I'm curious, What brand of Lithium did you go for and how did you solve your dc alternator charging regulation, dc to dc charger or regulator controller?

2 x 200Ah victron smart.
I rewired the internals of the alternator to be externally regulated and added a balmar regulator.
The engine start battery is lead/acid and is charged by a totally separate alternator, if it weren't I would use a dc-dc charger.

 

[geem]

Combined bulk and absorption time is typically 5to 7 hrs regardless of banks size. With a larger bank you can complete the bulk stage faster but you will have a longer absorption period to get to float. The only benefit of the larger bank is a shallower depth of discharge, so in theory longer battery life. In addition the larger bank over its life can lose some of its capacity without causing problems, in as much as you have spare Ah available. The problem is that the bank cost a lot more in the first place. From my experience of living aboard and friends doing the same, a bank of 8 deep cycle batteries would typically last 8 years of hard use in the Tropics. If you half the bank size you seem to manage 5 years but they cost half as much. A new bank every five years seems to make more sense

 

[jac]

My personal preference would be the extra battery. You already have the wiring set up for it and the physical space so no real effort to have 3. You could drop to slightly smaller batteries to make the physical aspect of lugging them around easier ( and reduce cost), 3 increases your redundancy ( 1 knackered battery only takes out 1/3 of your capacity) and of course for that one time when you get caught out at night, water in the fuel, poor visibility and just wanting to run all your electronics your systems will stay up for much longer, even a cloudy day or two will have less impact.

 

[Stemar]

It is only in retrospect that I realise how much I used to worry about state of charge, and how little I worry about it now.

I could say the same about fitting solar power. Our boat lives on a mooring, but is always fully charged when we arrive, and the panels pretty much keep up with our day to day usage in summer.