Keith-
I agree---active management and balancing during discharge has its merits.
The term commonly used for the arrangement you have is “buddy pairs”---or in your case, buddy triplets?
Where you have mS(nP) configuration like you have---i.e. “n” number of batteries in Parallel, then “m” of these groups in series, you indeed can get some automatic balancing benefits in charge/discharge. For lead acid batteries, this isn’t usually a problem, though if you were to have a cell short, the adjacent batteries would attempt to dump lots of current into that battery and cause potentially a lot of power, heat and problems to occur. But lots of folks in the EV community have done buddy pair arrangements for years and have spoken highly of its utility. Others don’t like it. A bad battery won’t be as easy to detect is another issue and it’s possible that that weak battery could overstress and help lead to early demise of its buddies.
Warning: While it is generally considered okay to buddy lead acid and it is considered perfectly acceptable to buddy Lithium batteries (so long as a good active battery monitor is across each buddy group, you NEVER want to buddy up batteries with NICKEL chemistry---
This is because that at the peak of charging, these batteries “peak” in voltage, then drop in voltage quickly as current continues to flow. In a buddy pair arrangement, the first battery reaching that peak would appear as a lower resistance than its buddies and not only get fed a greater percentage of the charging current, but as its voltage drops, it will suck even more from the buddy and go into thermal overload or worse.
So don’t buddy pair anything with nickel chemistry.
Another active balancing idea for discharge is to have a DC-DC converter that can draw from the full pack, then have its output go to the weak battery in your string. Use a current-limiting, 48v input DC-DC and have its output set so that it will output a constant voltage set for the charging peak. This could be engaged after your pack is charged and when you start to go out cruising. During discharge, that weak battery would get a boost from the entire pack up to the limit of the current limit. Better perhaps would be to have the DC/DC converter setpoint track the pack voltage such that its output is always the appropriate fraction of the pack voltage.
I’ve built one of these, but haven’t tried it yet.
-Myles
From: electricboats@
Sent: Sunday, May 23, 2010 12:37 PM
To: electricboats@
Subject: Battery Balancers [was: [Electric Boats] Re: QUAD BATTERY CHARGERS FOR A 48 VOLT BANK]
Myles,
I am still struggling through understanding batteries, but after struggling with a sick pack for a couple of years I think what we really want is a balancer that works on discharge.
Even if you equalize all the batteries to the same voltage - that doesn't mean they will perform equally on discharge. A sulphated battery will have greater sag and internal resistance and as the pack discharges the delta between it's voltage and the other batteries in string will get higher and higher. Eventually, it will get into the cell-reversal range.
So I think we need the Anti-Lee Hart shunt - one which open connections to good batteries when a weak one is detected and that current then used to boost the weak ones.
Somewhat by accident I think I have this - my batteries are arranged as 4 banks, 3-batteries in parallel for each (node?) in the string. If any one battery is weak, the other 2 in parallel ought to be holding it up to their combined voltage (right? I hear stories about odd current paths and wonder..). So it's the average of the paralleled batteries which "appears" in the combined string voltage. The odds of a single battery taking the whole pack down are reduced.
Similarly...
The alternate wiring - that of 3 48V strings in parallel - would not be able to do this as the good strings wouldn't "see" the weak battery since the strong ones will just take a slight excess charge (the charging problem all over again).
I have seen some "laddering" techniques where the 3 parallel strings are cross-connected at multiple points (vs just the ends) - this would seem to be another route to the same effect (won't work as well on my boat due to cable-length issues)
With occasional re-balancing this seems like it ought to work, but there are people like yourself with way more actual knowledge (vs guesswork) thinking about this on the EV/Solar forums.
So I guess what I am asking is...critique this method of wiring. Is there a benefit to parallelism within the serial strings? Or is it really causing problems due to weirdness that I can't foresee?
Thx
-Keith
--- In electricboats@
>
> As an alternative to multi-chargers:
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