Upgrading - Add 2 new batteries with 2 3year used or buy 4 new

[casetorres]

Hi,

Would like some guidance.
I'm adding100-200 watts to my 300 watt solar system on my van. The current system has 2 6 volt AGM batteries ( 3 years old).
Should I buy 4 new batteries or can I buy two and add to system?

Thanks much, Case

 

[AmargosaWayne]

Depends on the state of the old batteries, three years of heavy usage, light usage, or average usage?

Number of deep cycles, always fully charged? Ambient temperatures. Variables.

Better to have four of the same. The older ones may hold the newer ones back, so to speak.

Maybe buy two new, then check the older ones against the newer ones. Not much difference, then maybe good to go.

 

[frater secessus]

Here's how I would handle it

1. assure the controller is set up to match the existing battery manufacturer's charging recommendations
2. if the bank is still struggling, add the panel as planned. If identical they can go on the same controller. If not identical panel, I'd put them on a separate controller.
3. see what happens with the existing bank. The bank should "act bigger" with more panel, as it will get more current (which AGM craves) and the additional panel will help carry daytime loads.
4. if the bank is still struggling, for ~$50 install an isolator. Massive current for the AGM, and "free" charging anytime you drive
5. if the bank is still struggling, replace PWM with MPPT if not already done. This can be done for ~$180 for 400-500w of panel.
6. if the bank is still strugglling, replace the old two with new two ones, or rethink loads, or rethink battery chemistry. Example: I have light loads and replaced my GC bank with a single 100Ah LFP. Meets my needs 100% and is much easier to charge. Partial state of charge is not an issue with lithium chemistries.

Reasoning: IMO most lead-chemistry battery issues are the result of insufficient charging, not insufficient capacity. I'll guess maybe 10% of all vandwellers charge their lead batteries to spec, leading to "battery murder". The steps above give the existing bank a fighting chance with the least amount of money. If bank is already toast then we will have charging figured out for the next set.

Increasing solar slightly but doubling bank Ah would almost certainly make matters worse. Panel-wattage-to-bank-Ah ratios are crude rules of thumb, but can help us asssess proposed changes. Consider these ratios, assuming solar-only charging (no assist from the altlernator, shore, generator, etc):

• right now: 300w:~220Ah, or 1.36:1 <- probably inadequate
• increased solar on same bank: 400w:~220Ah or 1.82:1. <- possibly adequate depending on use and geography
• increased solar on same bank: 500w:~220Ah or 2.27:1 <- probably adequate depending on use and geography. Note: I ran my GC batts at 3.41:1 and got slightly more than rated cycles out of them to 50% DoD. I consider 3:1 to be a good target for ensuring sufficient charge in variable conditions. That would be ~650w for your GC bank.
• increased solar on doubled bank: 400w:~440Ah or 0.91:1 <- severely inadequate, really going backwards here. Recipe for battery murder, only now we are killing 4 batteries instead of 2. $$$.
• increased solar on doubled bank: 500w:~440Ah or 1.14:1 <- inadequate, worse than the 1.36:1 you have right now

Adding another form of charging (alternator, typically) can make a big difference in how much panel is required to keep a given capacity of lead bank happy. I did some crude math comparing the amount of panel needed to charge 100Ah of lead from 50% DoD in representative areas, with different controllers, with and without alternator assist. The math is laid out so you can run more specific numbers for yourself. Note that your bank is ~220AH (guessing) so the 100Ah numbers in that article will need to be multiplied by 2.2.

 

[DuneElliot]

Batteries should always be of the same brand, same age, same capacity. Adding new ones to an older system means that the old batteries will be a drain on the new ones and cause problems.

 

[maki2]

Of course you could keep the new and old on different systems. My friend's boat is set up that way. He has a switch setup he rotates for charging and distributing the power from them. More wiring and monitoring of battery status to do but it is feasible.