I have a small solar set-up and was about to wire in a second battery and increase my available amperage. I just read an article about only connecting batteries of equal power, voltage (which I understand) and AH (which I was surprised). I hadn't realized that AH may also play a part in this. Can someone tell me if this is true? I have 2 older marine batteries and I change over the wires from the charge controller to recharge them separately. My thought was to wire them in parallel, let them charge together and increase my amperage at the same time. First one is 12 vdc / 80AH 2nd one is 12vdc / 60 AH I also have a 12 vdc / 160 AH that will be coming out of the boat soon and was hoping to hook that in as well (once I pick-up another panel).

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110100101109 years ago
batteries tend to do nasty stuff when connected in parallel. stuff like discharging into each other and wasting energy this way use separate charging diode for each battery or charge them at different times when in use try to wire each to an independant circuit the problems sure can be overcomed with some extra electronics - then you can use them whatever way you want
olddawg (author)  110100101109 years ago
I like the diode idea, I hadn't even thought of that. Put the weakest in front and not allow back flow. Can I now assume that the charge controller should be hooked up to the first battery on one side & the last on the other? With the diodes and connecting to the first battery, it would only read the first? Thank you for your help!
i thought of a simple charging circuit made of computer power supply + resistor + diode. one that charges as long as its powered in and is not smart or automatic you connect the batteries 'equally' (in parallel) with independent diode and resistor for each battery its up to you to decide when a battery is charged enough and disconnect it such 'equal' connection may make problems on an advanced charger. when it tries to see what voltage is on the battery it gets 0 (cause the diodes block the way from the batteries back) so it may keep charging the batteries forever it also can get to the point when only one battery charges - the one with lower voltage in this case 'unequal' charging like you suggest seems better solution i think the charger will see the first battery but im unsure. for example if the weak battery begins to rise in voltage when its close to full charge the charger may stop and it'll start wasting itself into the other battery another option is that the charger sees normal charging current (that the second battery draws thru the diode) and keeps charging and overcharging the 1st battery as long as you dont overload the charger and diode i think its aceptable to 'fill' the batteries but you need to finish the charging of each one on its own all this is guess. i never tried any of it and never used smart charger. i allways used a large transformer and later hacked computer power supply as charger
olddawg (author)  110100101109 years ago
I had thought of a possible 'overcharge'! My thought was based on amps needed, amps supplied. Thank you for the input. I have to think this out a bit....
smart chargers are built to prevent overcharge when connected to one battery connecting 2 together (in whatever way) sure confuses the charger and it may do something wrong (depends on how does it estimate the gharge of the battery) if the charger has a led or buzzer etc that starts when a battery charges fully then there is a good solution build a circuit with relay that connects one battery at a time use the led on the charger (add a socket in the back of the charger that is connected to the led) as control for the circuit when it lights up a relay changes the battery and automatically starts charging the 2nd one
olddawg (author)  110100101109 years ago
Correction; I had NOT thought of a possible overcharge. And once again, I thank you for your input. These are the reasons I come here first!
Sandisk1duo9 years ago
discharge all batteries, then connect them while they are discharged, then charge them
NachoMahma9 years ago
. If they are all the same type of battery (eg, lead-H2SO4), you should be able to do it, but you won't get level charging from battery to battery, which can lead to an over-heating problem.
. I'm with LinuxH4x0r - try it and see (assuming the batteries are all of the same type). Just make sure to closely monitor the battery and charger temps.
olddawg (author)  NachoMahma9 years ago
Thank you for your comments! They are the same, basic lead acid marine batteries, just of different ages and AH. I will give it a go, but I believe I will add with the diode idea. I don't see any down side (at the moment), I do see positives! Let me know if you have any thoughts on this.
The only issue you'll have with a Diode is there is a loss of 0.7 VDC over the diode and with every amp you pass through it you'll be losing watts to the diode... And with that said there is protection built into your charge controller and most solar panels have a diode protection built into them, and adding more and more diodes will waste power and may even reduce the voltage to a level that you may not be able to obtain a full charge on your batteries.... And your batteries are fairly close and this is not a large wattage system with lots of cells involved so you could just go for it and wire them all together... I would recommend taking the batteries in and having an Equalization charge done on them especially if it's lived on a alternator for most of it's life, this should bring all the cells in the battery up to the same voltage levels and there will be less self discharging.. (( well worth the few bucks they charge to do it ))
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