Battery Bank Size and Charge / Discharge

[felixqld66]

Being new to managing a battery bank and inheriting a poor one just under 4 weeks ago here are some questions from a newbie. Given that I have no electrical knowledge this is a steep learning curve.

Here is the situation I inherited

The start battery was replaced by previous owner after my purchase prior to me taking over the boat old one had failed. It was poorly reconnected and all wiring to all batteries removed.

Start battery wet 12v 100ah new
2 x AGMs 150ah offline at the moment due to the system being ripped one reads 6.8v the other 9.2v
A 120w solar panel feeding into a CONERGY SCC1D Vision Charge controller I assume cannot find manual on net will have to contact Conergy

Here are my questions

1. Could the failed start battery pulled the AGMs down to such low volts and are they stuffed?
2. Then being offline for 3mths left them is a poor state?
3. I have to run a plotter,4kw radar wind instruments hydraulic autopilot and an Isotherm 3700 fridge in the tropics..... I am very frugal with lighting and power usage...happy to go to bed early n get up early. I figure I need a bank of about 400-500ah but to charge that up I am concerned that 120w solar will not charge me up enough. I am yet to work out my rate of discharge as I am getting my head around this electrics stuff.
4. Some of my reading suggests that a larger bank is better because you draw less from it overall but them other sources say that larger is not better????
5 Is it sensible to get a small Honda or Yammie generator and a 240v smart charger for topping up the battery as I don't like the idea of running the diesel just to charge battery ok if it is being used to motor/motorsail

Generally very confusing from a newbie when you are trying to get your head around it from books. I learn best when I sit down over a topic/ object with someone who knows what they are doing.

I head back to the boat in Thailand in late January to sort it out and provision to start cruising.

Any help appreciated

Cheers Alex

[noelex 77]

Congratulations on the new purchase. No doubt you will have lots of questions as you sort out the boat. That’s what the forum is for.

I am afraid your AGM batteries are likely to be dead. With those voltages there is little hope that they can be rescued.
It is likely they died of neglect / old age. Most yachts are wired so that the start and house banks cannot discharge into each other.
A larger battery bank is always better, but the main thing is you have to replace the power you take out. Batteries only store power, they don’t make any. 120W of solar is not a lot and will probably not be enough to run the fridge, which will be your largest power consumer.
A petrol generator is one option to produce more power, but I would suggest you live with the boat for a while before you make any alterations. If you can fit more solar that is often the better long term solution.
A energy budget would be a good idea, but you may need to live aboard to be able to measure your power usage.

[felixqld66]

Thanks for that I will have 13 wks on the boat when I get back to it in January so I should be able to get a handle on lots I hope. I do have a couple of sighting options for extra solar panels so I will look into that.

Cheers Alex

[motion30]

Those portable generators are popular and a good idea in my mind Your batteries may go dead but at least you can charge them given time

[DeepFrz]

Looks to me like the PO (or someone else) ripped you off and stole the batteries, replacing them with old ones. What was he doing on your boat after the sale anyway.

[djmarchand]

Alex:

Here are a few things to consider:

If you are frugal with lights and your main DC load is refrigeration, then you will probably use 60-100 amp hours per day.

A 120 watt solar panel will typically produce 40 amp hours of power on a sunny day. Much less on a cloudy day.

So you have two choices: more solar panels or a genset- the Honda or Yamaha that you mentioned.

If you choose the genset, then you probably want the 2000 watt model that can produce 1800 watts continuously. You will also need a higher output battery charger to convert the AC into DC that your batteries need. So replace your current charger, but don't get one any bigger than 50 amps or it will overload your genset. A smaller one will require longer running.

IOTA is a good and inexpensive brand of chargers.

David

[felixqld66]

Originally Posted by djmarchand

Alex:

Here are a few things to consider:

If you are frugal with lights and your main DC load is refrigeration, then you will probably use 60-100 amp hours per day.

A 120 watt solar panel will typically produce 40 amp hours of power on a sunny day. Much less on a cloudy day.

So you have two choices: more solar panels or a genset- the Honda or Yamaha that you mentioned.

If you choose the genset, then you probably want the 2000 watt model that can produce 1800 watts continuously. You will also need a higher output battery charger to convert the AC into DC that your batteries need. So replace your current charger, but don't get one any bigger than 50 amps or it will overload your genset. A smaller one will require longer running.

IOTA is a good and inexpensive brand of chargers.

David

So David it is hard work for me getting my head around this battery business so if i am shooting for a 400ah bank use between 80 -100ah, panel puts back 40ah at best how long will the 2000w gen need to run to top up the bank and with what charger do you assume it is with?

Also when you say replace your current charger are you referring to the conergy ssd 1 vision that is in the boat know or are we talking a separate charger for when the 2000w genie is running?

Thanks Alex

I am going to go wet cells to replace the agms