Elijah came with a pair of 12v 120A/hr AGM batteries in the house bank. Testing showed one of those batteries to be completely DOA, while the other is bulged and suspect, thus the decision to replace the house  battery bank. Normally I would recommend using LifePo4 batteries for a catamaran, but as I happened to have a reasonably large set of Lead Acid batteries hiding in the garage, I decided to go with the older technology.
Lead acid has a number of disadvantages and issues in use for a house battery, not least of which, for a catamaran is weight. My batteries are a set of Leoch FFP 6210’s – 6v Trojan copies, rated at 240A/hr ( 20 hr rate) . The weigh in at 26Kg each.  Now the original house bank was rated at 240A/hr which sounds reasonable until you start looking deeply at Lead acid technology and Peukert’s law.

The Peukert Law expresses the efficiency factor of a battery on discharge.  W. Peukert, a German scientist (1855–1932), was aware that the available capacity of a battery decreases with increasing discharge rate and he devised a formula to calculate the losses in numbers. The law is applied mostly to lead acid and help estimate the runtime under different discharge loads.

The Peukert Law takes into account the internal resistance and recovery rate of a battery. A value close to one (1) indicates a well-performing battery with good efficiency and minimal loss; a higher number reflects a less efficient battery. Peukert’s law is exponential; the readings for lead acid are between 1.2 and 1.5 and increase with age. Temperature also affects the readings. Peukerts law describes the charge and more importantly the discharge characteristics of lead acid batteries. Essentially, all lead acid batteries have discharge inefficiency which means the capacity drops as the rate of drain rises. This factor is refereed to as Peukert’s constant, and in the case of  flooded cells like mine is typically around 1.2  – 1.6 (mine exhibit a value of around 1.25).

In general a 240A/hr battery is rated at the 20 Hr discharge rate. This means you will get the rated capacity at a discharge rate of 240/20 = 12 amps! anything more than that and the capacity of the battery drops (that’s not good).
To add to your woes, lead acid is actually only able to reliably utilise around 50% or so of the rated capacity. Some deep cycle batteries can be discharged more deeply, but 50% is a good average to work with.  Thus our original house battery bank could reasonably be expected to deliver a miserable 120A/hrs at no more than 12 amps (240A/hr bank), or assuming all of the cabin lights are on (the original halogens) – only around 10 hrs of operation. Actually the maths is not that simple.

 

n = Peukert’s constant

C1 = Capacity at 1^st rate

C2 = Capacity at 2^nd rate

R1 = hour rate at 1^st C rating

R2 = hour rate at 2^nd C rating

 

Calculating what battery will run your load

Here is gets complicated.  In this formula we are trying to solve for C, the Rated capacity at a 20hr rate.

 

This would indicate that in order to support a 12A load for 24 hrs would really require a 544A/hr bank. Using my batteries that would mean 6 x 6v batteries, weighing in at around 156kg!!!
Hmm, maybe I will need to re-think using these batteries.

Battery Power Budget.

So, how much power do we  really need?  Perhaps we should look at what power draw we have on the boat:

Fridge: Approx 35 watts = 2.8 amps

Domestic Lights: 7x 5 watts = 35 watts = 2.8 amps

Bilge pumps: 10a

Fresh water Pumps: 7.5a

Water Heater:  40w in circulation pump = 3.2a

Chart Plotter inc radar: 5-6 amps

VHF: 0.7a RX 3.5A TX.

Nav instruments: 0.5a typical

TV: around 3 amps

Sterio: 2-5 amps (depends on how loud )

 

Our power budget is approximately 2500watts / day, or about 186A/hr / day, with a maximum draw of around 23 amps.  This is not a really heavy draw boat, but it does put the house bank into perspective. In lead acid, we would need a house bank of around 500A/hr. New these batteries run at around £130 each A 4 cell bank at 420 A/hrs comes in at a list price of around £520 – not exactly cheap these days. 6x batteries give us a house bank of 630A/hr but at a cost of £780, and a whopping 156Kg . Fortunately for me, I have such a bank sitting in the garage on trickle charge.

Moving to LifePo4 would save a huge amount of weight, but at a cost. Our 186A/hr budget could be met by a 220a/hr bank at 80% depth of charge. Lifepo4 has a number of important advantages in addition to much lower weight. Lifepo4 does not sag under heavy load the way Lead does, and it is safe at 80-90% depth of discharge. charging is also vastly improved over lead acid, LifePo4 will absorb power pretty as much as fast as you can supply it with near 100% efficiency (unlike lead acid). This means that charging can be very fast, but is really hard on charge sources, particularly alternators. in general, alternators are quite well tuned to lead acid charge profiles, ie: high initial charge rate, tailing off rapidly as the battery hits float zone.  this means that alternators run hard for a fairly short length of time. LifePo4 of the other hand never reaches as float level and thus absorbs everything the alternator can throw at them. The result is 100% alternator load at all times while charging. This can lead to alternators overheating in short order unless care is taken – we will explore LifePo4 charging and management at a later date.

At the moment, large capacity LiFePo4 cells are still quite expensive, but I do currently have access to some very lightly used CALB 210 a/hr cells – at £120 each, I would need 4x so ££480. I also have  access to some 26650 based packs running at 3.2v 120A/hr for £80 each. With these I would  need 8x iin order to get 240A/hr at a cost of £640 – not such a good deal, but definitely not bad when compared to the price of Lead. Indeed, not so long ago, the purchase cost of LiFePo4 was considerably more than the cost of LA batteries. These days the price has come down considerably and with a little hunting, and imagination, is now very comparable and sometimes cheaper. If going for new batteries, then LiFePo4 is still not terribly cheap. 260A/hr Winston Cells are available for around 275 euros each, so a bank would come in around 1100 euros. That sounds high compared to LA capacity, but a better view should be to price ‘usable’ capacity rather than rated capacity.

6x 6v 260A/hr lead acid = 720A/hr@ 12v.
Usable capacity (low draw) approx 50% = 360A/hr
Price = £130 each = £780.  ( approx £2.16 per usable amp/hr)
Weight = 6*26kg = 156Kg !!!

 

 

4x 300A/hr Winston LiFePo4 = 300A/hr  ( Link Here)
usable capacity (90%) = 270A/hr
Price = (todays rate) £285 each = £1140  (approx £4.20 per usable amp/hr)
Weight 4* 10kg = 40Kg ( less than 2x LA)

 

At the moment LA is still a winner from an initial cost perspective, but do remember that LiFePo4 is far better at high loads, doesn’t sag, recharges MUCH faster and lasts far longer. In fact it is generally accepted that LiFePo4 will last 3-5 times longer than LA for house loads unless the draw is very light indeed.
If I was buying, I would look for surplus LiFePo4, or even new, as overall it is a far better solution. Indeed, when it comes time to replace the house bank again, I will be doing just that, but for now, I will use the LA because I already have them sitting in the garage doing nothing. I certainly don’t expect them to last or perform as well as LiFePo4, and will be doing all I can to minimise draw.