Not Balancing Three 12v Right

BFL13 wrote:
That smartgauge is what I was trying to say in the OP where I had three batts done as though two.

He says the middle battery discharges less than the end ones, but I didn't measure that. I saw the middle battery lagging on the recharge by its SG showing it not as full as the end ones.

I don't know why the middle battery doesn't come back on the recharge where it takes fewer amps, since it also provides fewer amps on the discharge. Maybe while camping there is more time for them to even out on the discharge side so the recharge is uneven right after, allowing no time (days?) for things to even out.

After reading the smartgauge article again (I thought I had read and understood it. LOL), I agree that what you observed is probably valid.

I'm not as surprised, as you, that you would see the same thing on the recharge as you would observe under load contribution measurements.

Thanks westend. Glad to know it is not just my weird set-up that causes this.

How can you tell with the AGMs? (No hydrometer test.) What do you do to fix it when you do spot that?

FWIW, I have three 12V AGM's connected as per the #2 balanced connection posted and I noticed the same phenomena, the middle battery charges less.

That smartgauge is what I was trying to say in the OP where I had three batts done as though two.

He says the middle battery discharges less than the end ones, but I didn't measure that. I saw the middle battery lagging on the recharge by its SG showing it not as full as the end ones.

I don't know why the middle battery doesn't come back on the recharge where it takes fewer amps, since it also provides fewer amps on the discharge. Maybe while camping there is more time for them to even out on the discharge side so the recharge is uneven right after, allowing no time (days?) for things to even out.

OnaQuest is right in warning this could be a bogus observation, not to be applied to anyone else's situation, but it is what I got with my messy set-up.

I did not take SGs while camping on completion of a 50-90 to see how the batts shared that. It was after getting home with the batts down and plugging in to let the converter bring them up (it is slow). By next day they are almost full, and that is when I took the SGs. I have seen this every time, so it was not just a one-off.

That set of SGs is to tell me if they need the high voltage finishing charge to get them to 100% which they always do need. The converter can't do it-voltage too low. I then have to do each of the three separately as mentioned with a charger that has the high voltage into the 15s. Then I can bank them as before put them all on the converter for a Float. It takes two or three days after getting home before the batts are all done up and on float.

(Leaving them on the converter for weeks instead is a disaster--the SGs do not come up to full. Got to get them into the 15v range like US Battery and the others say you do before going on the float.

The Float voltage is then able to keep all the batts at their baseline SGs despite the unbalanced set-up. Mex explained this was ok in a previous thread, as long as the battery taking the smallest share of the amps is still getting enough to overcome its own self-discharge.

Just one more reason why hydrometers should be outlawed ! :)

Kidding aside, I don't need AGMs, but BFL does :)

Give this link a read:

http://www.smartgauge.co.uk/batt_con.html

Use the most complex Method 3 for a perfect balanced parallel connection of any number of 12V batteries (even though the example photo shows 4 batteries). Use the simpler Method 4 for a perfect balanced parallel connection of four 12V batteries. I use Method 2 for a perfect balanced parallel connection of just two 12V batteries (this method is not perfect for more than 2 batteries, however). The goal is to have all current from every battery flowing the exact same "overall wire path distance" from it's positive terminal, to the load, and from the load back to it's negative terminal .... as every other battery in the bank. Of course all of this wiring should using as large a gauge cabling as you can afford and fit.

No I did not switch batteries around, they are not all identical batteries (all are true deep cycle though for antimony) and my wiring is not equal so it is not science. Could be a false interpretation I guess.

PT did warn me here in the past that the middle battery would be undercharged by using this unbalanced method pretending it was a bank of two, so I figured that was the explanation. (I know the right way with buss bars and equal length wires, but can't do it that way)

Perhaps if you have a clamp on ammeter you could spot if one battery is taking fewer amps than the others when the whole bank is being recharged.

After you do some 50-90s camping and go to shore power, then next day when they are nearly full, check their SGs. If one battery's SGs are lower than the other two's, then you know. That's what I get. I don't know if they would even up after several days either. I go right ahead and get them all to 100% individually now that I have shore power.

BFL13 wrote:
This post is just for info in case anybody else has a similar situation

I have learned the truth about wiring up three 12s as though they were two 12s.

With the load and charging pos and negs on the outer batteries, I have no measurements for discharging, but confirm that when recharging, the middle battery does not reach as high an SOC. It is the same voltage as the others of course, but has a lower SG for measuring SOC after the recharge is done.

Eventually, someday, the electrons might all even out and so the SGs might even out, can't say. Doesn't matter , can't/don't want to leave them undercharged that long.

This doesn't matter when doing fours days camping in a row, then going home and getting each set of batteries up to full baseline SG separately before putting them all on a Float.

(It is worth banking them all for those four days to get the Peukert advantage. Eg, I was able to go two days with them all banked before needing a recharge, where before with them in separate banks, I needed a recharge every day. Confirmed Peukert advantage!)

But, now heading into solar charging time for five months off-grid, I now see a peril in leaving the three 12s all hooked up as though two 12s, where the middle one will never get up to speed for months.

I can't wire them up "properly" for three 12s (various reasons) so I am going back to having two banks and properly balancing each bank for recharging.

I know that method works well from last summer, and with it being warmer and longer days, there is no daily AH need for the Peukert advantage doing 50-90s like in winter, no solar. I will be doing more like 85-97s on solar daily. I can't leave the middle battery undercharged for months!

Different situation, different set-up.

Are you sure that what you observed was NOT just a difference in batteries? Did you try the same experiment by moving one of the outside batteries to the middle? There is NO logical explanation for what you observed if all battery cable paths were the same length through any given battery.
edit; Last sentence was reworded to be more understandable.

That would be me I guess... I just used equal 4AWG cable lengths in my two banks. I have two 12VDC Batteries in parallel in one bank and the second bank only has one 12VDC battery in it. Both banks are fed by my PD9260C converter/charger thru my BLUE SEA four position battery switches. All like cables are cut to same length...

My two banks are wired up like this. I do know I am seeing 52-53 AMPS being drawn from the three batteries with my DONUT current sample when using my PD9260C 60AMP converter/charger in the initial 14.4VDC BOOST mode.


I really haven't measured each leg but my three batteries seem to have good performance when I run them back up to their 90% charge each day after they have run down to 12.0VDC over night.

Are you saying you have a better way for me to wire these up ??? Running the BATTERY NEG terminals to FRAME GROUND with equal length 4AWG cables was the easiest thing for me to do at the time.

I will probably get to start upgrading my old batteries this season. These have been in service since 2009. I'm wanting to start my SOLAR PANELS on my POPUP ROOF this year as well.

Roy Ken