Can I mix battery sizes ?

[Philandviki]

Hi . I am brand new with solar. I have 90 watt panels. I have them charging 1 , 35 amp hour battery and another much larger marine battery connected together. Is there a disadvantage to this ? It seems like based off my calculations that I should be able to run things much longer than I am . Should I disconnect the smaller battery ? Will this make a differance ? They are both 12 volt .
Thank you

[SunEagle]

Hi . I am brand new with solar. I have 90 watt panels. I have them charging 1 , 35 amp hour battery and another much larger marine battery connected together. Is there a disadvantage to this ? It seems like based off my calculations that I should be able to run things much longer than I am . Should I disconnect the smaller battery ? Will this make a differance ? They are both 12 volt .
Thank you

First. Charging two different size batteries wired together is not a good idea. Each battery requires a certain amount of amperage to properly charge and you have no way of sending the correct amount to each battery so one may get less than what is needed and the other may get too much.

Second. Depending on how "big" your marine battery is that 90 watt panel may not be big enough to charge by itself. What is the AH rating of your marine battery?

A good ratio of battery charging is C/8 to C/12 where C = the Ah of the battery. So a 200Ah battery will be 200/8 = 25 amps and 200/12 = 17 amps. That 90 watt panel can probably only put out 4.5 amps which is enough to charge your 35 Ah battery but not one bigger than 55Ah.

[Philandviki]

First. Charging two different size batteries wired together is not a good idea. Each battery requires a certain amount of amperage to properly charge and you have no way of sending the correct amount to each battery so one may get less than what is needed and the other may get too much.

Second. Depending on how "big" your marine battery is that 90 watt panel may not be big enough to charge by itself. What is the AH rating of your marine battery?

A good ratio of battery charging is C/8 to C/12 where C = the Ah of the battery. So a 200Ah battery will be 200/8 = 25 amps and 200/12 = 17 amps. That 90 watt panel can probably only put out 4.5 amps which is enough to charge your 35 Ah battery but not one bigger than 55Ah.

I just disconnected the smaller battery . My marine battery is not that great. It's only rated at 105 amp hours LOL!! Could my 90 watt panels charge this battery you think? Also if I am using and inverter and have a tv running that's rated at 60 watts how long do you think I could run it for before the inverter shuts down ?

[ILFE]

I just disconnected the smaller battery . My marine battery is not that great. It's only rated at 105 amp hours LOL!! Could my 90 watt panels charge this battery you think? Also if I am using and inverter and have a tv running that's rated at 60 watts how long do you think I could run it for before the inverter shuts down ?

By the way you have written "panels", twice now - plural, I can't help but ask. How many 90 watts modules (pv panels) do you have connected? Just one? Two or more? Just making sure here, so we are all on the same page.

[SunEagle]

I just disconnected the smaller battery . My marine battery is not that great. It's only rated at 105 amp hours LOL!! Could my 90 watt panels charge this battery you think? Also if I am using and inverter and have a tv running that's rated at 60 watts how long do you think I could run it for before the inverter shuts down ?

Your 90 watt panel is too small to properly charge your marine 105 Ah battery. You will need about 200 watts with a MPPT charger.

If you run the tv until the inverter shuts down you will start hurting that battery. You really should not discharge a deep cycle battery more than 25%.

A 25% discharge for the 105 Ah 12v battery is roughly 300 watt hours. So maybe 5 hours at best to run your TV. The bad part is that you 90 watt panel will not really be able to recharge it back to 100% in one day so you would have less run time for your TV the next day.

hmmmm. I just noticed that also. So how many 90 watt panels do you have? All my statements have been based on only one 90 watt panel.

[Philandviki]

I'm sorry I'm really new here. I have 6 , 15 watt panels . Bringing the total up to 90 watts. So even adding another battery would not help. It's more panels I need even for the 1 battery.

[SunEagle]

I'm sorry I'm really new here. I have 6 , 15 watt panels . Bringing the total up to 90 watts. So even adding another battery would not help. It's more panels I need even for the 1 battery.

My suggestion is to use those 15 watt panels to keep your 35AH battery charged and then get yourself maybe a 200 watt panel, a 15A MPPT Charge controller to take care of that 105Ah battery.

I know having two systems seems strange but I can tell you that your "90 watt" panel system is not high quality, will not last long and won't charge your big battery.

Is is better going with a 200 - 250 watt grid tie panel and MPPT charger which will allow you to charge your 105Ah battery.

[Philandviki]

My suggestion is to use those 15 watt panels to keep your 35AH battery charged and then get yourself maybe a 200 watt panel, a 15A MPPT Charge controller to take care of that 105Ah battery.

I know having two systems seems strange but I can tell you that your "90 watt" panel system is not high quality, will not last long and won't charge your big battery.

Is is better going with a 200 - 250 watt grid tie panel and MPPT charger which will allow you to charge your 105Ah battery.

So I'm asking questions because I don't know and I respect your advise and opinions here. If the solar power was something I was going to use every two weeks or three weeks would it be better to keep the bigger battery hooked up giving it two or three weeks to charge then use it ? Or is it still better to just stick with the smaller battery with my panels ?
Thanks again for your input !

[SunEagle]

So I'm asking questions because I don't know and I respect your advise and opinions here. If the solar power was something I was going to use every two weeks or three weeks would it be better to keep the bigger battery hooked up giving it two or three weeks to charge then use it ? Or is it still better to just stick with the smaller battery with my panels ?
Thanks again for your input !

Using those 15 watt panels to trickle charge your larger battery is one way to use them but they will never be able to properly recharge it after one use. You would need another type of charger powered from a generator to get it back close to 100% and then trickle charge while you are not there. Without a way to properly charge your batteries will cause them to shorten their life. Almost better to charge the battery at home and then bring it with you.

If you want to use those 15watt panels then they will work with your smaller battery, but to be honest those panels will not really last long. If you want to run a higher loads that are too big for that 35Ah battery you should at least think and plan about getting a 200 - 250 watt panel and an MPPT charge control for the bigger battery.

[Philandviki]

Using those 15 watt panels to trickle charge your larger battery is one way to use them but they will never be able to properly recharge it after one use. You would need another type of charger powered from a generator to get it back close to 100% and then trickle charge while you are not there. Without a way to properly charge your batteries will cause them to shorten their life. Almost better to charge the battery at home and then bring it with you.

If you want to use those 15watt panels then they will work with your smaller battery, but to be honest those panels will not really last long. If you want to run a higher loads that are too big for that 35Ah battery you should at least think and plan about getting a 200 - 250 watt panel and an MPPT charge control for the bigger battery.

Thank you . I will upgrade at some point . This was just a small first step in solar. I will change to the smaller battery. Right now I'm just running a small lamp and phone chargers off the power but would someday like to be able to power like my tv or something usful .