Mixing Lead Acid starter battery with AGM domestic battery

[LeonF]

I know there has been discussion on this topic previously, but none of it seems to be conclusive. I have a new Beta 16 with a 40 ah alternator. No split charging system. I start on battery 1, use both when the engine is running, switch to 2 when sailing. Start batter is a Yuasa 80 ah, fairly new and retains a good charge. Battery 2 is older, non branded from Tayna Batteries. I feel it will need replacing soon, charges with mains charger to around 12.97v. I'm constrained on size because of the battery boxes and where they are located under port and starboard berths makes increasing the box sizes a big expense. Halfords sell a Yuasa AGM deep traction battery, 90ah which looks attractive, but would I encounter charging issues, especially when switched to both ? I don't have a fridge, heater etc, just chart plotter, auto pilot, depth sounder, but lots of lights etc. I had a Trojan battery when I first bought the boat some 20 years ago that lasted well, charged through a split diode sensor, but that has been taken out of the system with the new engine.

 

[PaulRainbow]

The charging profiles of SLA and AGM are close enough so as not to cause a problem.There are tens of thousands of production boats fitted with a mix of SLA and AGM, no problems.

Having the switch on both is bad practice (queue someone telling me they have done this for the last 200 years without a problem). Whilst it on both, a catastrophic battery failure could mean you lose both batteries. Separate switches is the best answer, with split charging. If you don't want to make that change, at least fit a VSR for split charging. You can then start on the SLA and immediately switch to the AGM, not having to worry about charging or forgetting to take the switch off of the "both" setting when you stop the engine.

 

[LeonF]

The charging profiles of SLA and AGM are close enough so as not to cause a problem.There are tens of thousands of production boats fitted with a mix of SLA and AGM, no problems.

Having the switch on both is bad practice (queue someone telling me they have done this for the last 200 years without a problem). Whilst it on both, a catastrophic battery failure could mean you lose both batteries. Separate switches is the best answer, with split charging. If you don't want to make that change, at least fit a VSR for split charging. You can then start on the SLA and immediately switch to the AGM, not having to worry about charging or forgetting to take the switch off of the "both" setting when you stop the engine.

Hi Paul. Thanks for your reply. I've spent a lot of dosh on the boat so trying to keep further expenditure to a minimum. How about if I just start on the SLA then immediately switch to the AGM without a VSR ? I also a have a very low output solar panel merely to keep it trickle charging while the boat is on its swinging mooring.

 

[PaulRainbow]

Hi Paul. Thanks for your reply. I've spent a lot of dosh on the boat so trying to keep further expenditure to a minimum. How about if I just start on the SLA then immediately switch to the AGM without a VSR ? I also a have a very low output solar panel merely to keep it trickle charging while the boat is on its swinging mooring.

If you switch straight away the SLA doesn't get charged. What does the solar panel charge ?

If this is the Halfords battery https://www.halfords.com/motoring/b...sa-active-leisure-battery-l26-agm-466918.html

It's expensive. I don't believe the claim from Halfords that it's deep cycle.

As you have fairly minimal requirements, perhaps something like this would be suitable https://www.bmstechnologies.co.uk/product/hankook-xv27-leisure-battery/

You could fit a VSR with the difference and still have change.

 

[vyv_cox]

I have the opposite. Red Flash 1300 AGM starter, now 10 years old, 3 x Winner Solar flooded lead acid domestic bank, 5 years old. I agree with Paul, I never use 'both' on my switch. If starting on the Red Flash I charge this for 30 mins or so, then switch to domestic. I have a dual PWM controller, so if the alternator is charging one the solar is charging the other.

 

[VicS]

Hi Paul. Thanks for your reply. I've spent a lot of dosh on the boat so trying to keep further expenditure to a minimum. How about if I just start on the SLA then immediately switch to the AGM without a VSR ? I also a have a very low output solar panel merely to keep it trickle charging while the boat is on its swinging mooring.

Simple. When you are ready to start the engine

• Select the starter battery
• Start the engine
• Keep the starter battery selected for long enough to recharge it. A voltmeter or ammeter will help to determine when this is done.
• Briefly select both then immediately switch to the house battery only ... Do not ever switch both off while the engine is running or you are very likely to blow the alternator diodes.
• Keep the house battery selected until you next need to start the engine or are leaving the boat.

The rule on the boat I crewed was that only the skipper switched the batteries.