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paralleling different batteries

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snowflake View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Topic: paralleling different batteries
    Posted: 19 June 2021 at 12:12pm
Hi

has anyone got a system that allows for connection of batteries of different types or charge states?

Been looking at schottky diodes and ideal diode controllers to prevent current flowing from one battery to the other.

Vishay 15V 80A, Dual Schottky Diode, 3-Pin D-61-8 VS-85CNQ015APBF | RS Components (rs-online.com)


any thoughts on either of these?

can the same be achieved with a suitable solar controller with multiple inputs or do these have limited current output?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 19 June 2021 at 12:22pm
Mixing batteries is generally not a good idea. What are the batteries you want to use? And for what? And how many? And what is the source for charging? A PV system, or mains? The solution will depend on all those factors.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 19 June 2021 at 12:42pm
My mates were running a large 12V rig off SLA batteries at a free party the other week. I think they started with 4*200Ah. When their batteries started dropping voltage people were bringing fully charged leisure batteries from their vans and connecting these direct to the old batteries. seems to me a lot of energy would be wasted in current between the batteries and you would instantly lose the higher voltage of the new battery.

I'm building a system that will use a LiFEPo4 battery but wondering if it wold be possible to connect a lead battery when necessary. Would have separate charging systems for the two batteries as dual systems seem to be very expensive and not necessary for what I'm doing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 19 June 2021 at 3:11pm
Ok, understood.

Then in your case I'd say that a decent sized schottky should be enough to protect the system if/when you switch in an extra lead acid battery. Or that board you reference as well, although who knows how reliable that would be.


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Phil B View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Phil B Quote  Post ReplyReply Direct Link To This Post Posted: 20 June 2021 at 12:49am
I would not put different model / make batts in parallel for any length of time, esp when charging. For a " get out jail" discharge only sure .
The charging settings for SLA / flooded / agm are very different from Li-Po ( no absorb / eq) and you could damge either if charged continuously on the wrong settings.
I don't know any solar controllers off the top of my head that allow multiple / different type batt connections. Mostly it's a single bank which you set the charge type for and the MPPT / PWM works around that ?

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Post Options Post Options   Thanks (1) Thanks(1)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 21 June 2021 at 2:25pm
Sure diodes are the device you need to stop power flowing where you don't want it to go. But at the currents involved, finding the right device is key.
 
I personally would not trust a 15v diode to not break down or at least have significant reverse current flow in a "12v" system, where the fully-charged voltage could be 14.5v. And they don't recover like Zeners so once you've burned through it, it's burnt...


Remember that diodes need cooling too; across that Schottky at full power, you are dropping 1v. So if your amplifiers are drawing 50A the diode will have to dissipate 50 watts. Not trivial! A device without heatsinking will quickly burn up. So you need to consider building "diode modules" that have heavy gauge in/out connections and the actual diodes mounted on a substantial heatsink. 

Additionally, remember that 1v drop comes off the available voltage at your amplifiers - so you will not be able to use as much of your battery capacity before hitting their low voltage cutoff - however you shouldn't really be regularly discharging lead-acid batteries below 50% anyway!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2021 at 11:59am
there are a few solar chargers that can charge two different battery types at once. and dedicated mains chargers can be used to take power from grid or generators when available.


Edited by snowflake - 23 June 2021 at 1:58pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2021 at 10:05pm
EDIT: deleted previous post which below replies to.

I didn't think about the problems of high current switches - duh! Either very expensive contactors or using MOSFETS to do the switching in the first circuit I drew (and I'd drawn the schottky bypass wrong). seen 200A mosfets for under £20 each...

second might work using an MCB to connect the lead battery. not sure how or whether to implement the current limiting. some LiFePo4 batteries are rated at 70A or more max charge current. maybe just get a very high amp rating schottky and some large gauge wires that can deal with the high current for a couple of minutes as the voltages equalise. usually will connect additional battery before LiFePo4 voltage has dropped too far below 12V so won't be an issue then anyway.


Edited by snowflake - 23 June 2021 at 1:59pm
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Phil B View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Phil B Quote  Post ReplyReply Direct Link To This Post Posted: 23 June 2021 at 10:45am
Well I stand corrected ! Never seen a dual batt controller and that Brand and maybe a different model might solve my mates truck dilemma ... 24v truck starter batts with 12v BIG leisure batts and lots of roof panels...

Thnx

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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 23 June 2021 at 2:31pm
I'm going to try connecting fully charged SLA through a schottky to a partially discharged LiFePo4 and measure how large the current between batteries is and how long it lasts for. will start off with LiFePo4 at 12V and then try at 11.5,11,10.5,10. if I'm seeing current >40A for more than a minute will look at a more complicated circuit though I'm struggling to get my head round how to do it. The below might work with suitably rated components but please don't anyone think I know what I'm doing and copy this layout uncritically.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote -arthur Quote  Post ReplyReply Direct Link To This Post Posted: 02 November 2021 at 9:44am
Vraiment intéressant
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