Charging batteries over winter layup

[davidmh]

This winter I am going to do some rewiring on the boat and it would be good to have the batteries connected, I usually take them home for the winter. I am wondering the best way to keep them charged. There seems to be two options. I expect to visit the boat at least once a week for 5 or 6 hours. I have mains electric when I am on board. There are 2 batteries of 100 AH
1. I have a 20W solar panel which can be switched to charge either battery, but not both, and this keeps the system topped up in the summer. Question would the panel get enough sun during the winter to do this, the boatyard gets sun from around mid to late morning onwards?
2. I have a mains CTek smart charger max 5Aamp which I can connect direct to the battereies. Would charging for 4 to 5 hours once every two weeks keep the batteries charged?
Note The boat has no built in shorepower system with battery charging
Any advise welcome
David MH

 

[Refueler]

You can buy small maintenance chargers that output about 1A and they stay connected to the batterys all the time. I use them in winter ... one for each battery ... they auto switch on / off so battery stays topped up.

Black Car Battery Charger Maintainer Auto 12V Trickle Hot For Truck Nice RV | eBay

 

[MontyMariner]

Switch the solar to the other battery after each visit. If that's not enough use your mains charger when you have power available.

 

[noelex]

Lead acid batteries typically self discharge about 1.25% per week when in good condition. This is temperature (and a few other factors) dependent. In winter in the UK the discharge should be less than this number. Around 2.5 Ah per week should be the most pessimistic prediction for your battery bank.

A 20 W solar panel be capable of replacing of replacing this in the conditions you describe. The main drawback is that the solar controller will initiate a new charge profile of bulk, absorption and float each solar day. When this is done to an almost 100% full bank it will slightly overcharge the batteries. If your solar controller has variable, user adjustable set points then these can be changed to eliminate this issue, but many cheaper solar controllers that are often matched to smaller panels do not have this facility. Nevertheless, the overcharging will only be slight and this is still a good option.

Likewise, your 5 A charger used once a week will be able to comfortably replace the batteries self discharge. Even if this slightly overcharges the battery (because they will be around 99% full when you start charging) this is only once a week.

So in summary, either option should be satisfactory.

I would check the battery SOC on each of your visits.

Note: This assumes there are no loads. You need to ensure that are no small parasitic loads. These can add up, particularly if you are relying on the small solar panel.

 

[PetiteFleur]

The Ctek 5amp charger should keep your batteries over winter fully charged - swapped over every 2 weeks. I have one on board which certainly keeps the 2 x 105ah domestic batteries charged when in a marina. When on my river mooring I have 120w of solar which again keeps the 2 domestic batteries and engine battery fully charged using a dual battery PWM controller with adjustable output, I use 10% for engine & 90% for the domestic batteries. MPPT controllers are now available which are more efficient.

 

[johnalison]

Even if you do nothing, you won’t run flat, though this won’t be ideal. I would go for whatever is easiest. I leave mine on a float and benefitted from being able to wait 9-10 years before replacing basic house batteries.