Testing 6V Deep Cycle Batteries
I had my 6V batteries load tested at an auto parts store and they failed. They were tested while connected in series and so were tested as a 12V system. I took them back to Costco and they were replac...
Forum Discussion
In response to comments in a couple of posts above regarding two 6 volt batteries in series:
"First: IF one battery fails "Shorted cell" now the other battery is being charged at a far too high voltage.. That causes damage"
1. Wouldn't that mean that after the next charging attempt the owner would notice that the resulting voltage of the series pair was still 2 volts low ... hence immediate action needed to check out the whole situation - before permanent/serious damage to the good 6 volt batterry?
"If the one battery fails "Open cell" or HIgh Resistance the other battery is not charged at all.Or not fully."
2. Wouldn't that mean that after the next charging attempt the owner would notice that the resulting voltage of the series pair was still low, so maybe no charging had taken place ... hence immediate action needed to check out the whole situation - well before permanent damage to the good 6 voltt battery from sitting around under-charged?
"If the older battery has LOST capacity then it may well end up reverse charged by the newer battery"
3. How could the good 6 volt battery reverse charge the bad battery backwards through the entire coach's (or exterior charger's) electrical circiutry?
"With batteries in parallel it's not as important they be identical"
4. IMHO it's the opposite of that, damage-wise ... with batteries in parallel one of the batteries with a shorted (or lowered resistance) cell can really, over a short time, damage the other good battery(ies). However from a capacity viewpoint, with batteries in parallel if one of the batteries has reduced capacity but still shows normal terminal voltage (I've had this happen more than once over the years) ... then the other good battery(ies) are not damaged - the total capacity of the bank is merely reduced by the under-capacity amount of the bad battery(ies). This is why one can make up a parallel RV battery bank of using batteries of different amp-hour ratings - they need not be all of the same capacity.
"We keep hearing how when one reaches full it will signal the converter to stop.. But the face is in parallel. When one reaches full THEY ALL REACH FULL"
5. Correct, but with more detail ... batteries in parallel are of course all connected together on the same cable and there can be only one voltage reading from this cable, hence the charger/converter "thinks" that is the voltage of the battery(ies) that it's charging and when this voltage reaches what the charger/converter "thinks" (according to it's design) is the right value, it stops with the charging. At that poinit all batteries will be full - including old ones at whatever capacity they are capable of according to their age and/or other factors that affect capacity over time.
"One simple and obvious example: if the bad battery had a shorted cell, then all other cells (including those in the good battery) would get overcharged when charging. In the other direction, it may be the case that the newer battery discharges faster than the old (lead acid batteries take a few hundred cycles to reach full capacity), so the cells undergo deeper discharges."
6. Correct on over-charging or deeper discharges of other cells in the case of a shorted cell anywhere in a series string. However, isn't a shorted cell way more rare than an open or high resistance cell (that's been my experience, BTW)? Even when a cell shorts, the reasoning in my comment in 1., above, may help to keep the good battery in a series string from being too badly damaged.
I'm still wondering if it's not an advantage having 6 volt batteries in series over 12 volt batteries in parallel - if one is astute/picky/experienced enough to know when only one of the batteries in a series string needs replacing because ... why replace all 6 cells (more money) in a series string when only 3 cells (less money) in a series string need replacing?
"First: IF one battery fails "Shorted cell" now the other battery is being charged at a far too high voltage.. That causes damage"
1. Wouldn't that mean that after the next charging attempt the owner would notice that the resulting voltage of the series pair was still 2 volts low ... hence immediate action needed to check out the whole situation - before permanent/serious damage to the good 6 volt batterry?
"If the one battery fails "Open cell" or HIgh Resistance the other battery is not charged at all.Or not fully."
2. Wouldn't that mean that after the next charging attempt the owner would notice that the resulting voltage of the series pair was still low, so maybe no charging had taken place ... hence immediate action needed to check out the whole situation - well before permanent damage to the good 6 voltt battery from sitting around under-charged?
"If the older battery has LOST capacity then it may well end up reverse charged by the newer battery"
3. How could the good 6 volt battery reverse charge the bad battery backwards through the entire coach's (or exterior charger's) electrical circiutry?
"With batteries in parallel it's not as important they be identical"
4. IMHO it's the opposite of that, damage-wise ... with batteries in parallel one of the batteries with a shorted (or lowered resistance) cell can really, over a short time, damage the other good battery(ies). However from a capacity viewpoint, with batteries in parallel if one of the batteries has reduced capacity but still shows normal terminal voltage (I've had this happen more than once over the years) ... then the other good battery(ies) are not damaged - the total capacity of the bank is merely reduced by the under-capacity amount of the bad battery(ies). This is why one can make up a parallel RV battery bank of using batteries of different amp-hour ratings - they need not be all of the same capacity.
"We keep hearing how when one reaches full it will signal the converter to stop.. But the face is in parallel. When one reaches full THEY ALL REACH FULL"
5. Correct, but with more detail ... batteries in parallel are of course all connected together on the same cable and there can be only one voltage reading from this cable, hence the charger/converter "thinks" that is the voltage of the battery(ies) that it's charging and when this voltage reaches what the charger/converter "thinks" (according to it's design) is the right value, it stops with the charging. At that poinit all batteries will be full - including old ones at whatever capacity they are capable of according to their age and/or other factors that affect capacity over time.
"One simple and obvious example: if the bad battery had a shorted cell, then all other cells (including those in the good battery) would get overcharged when charging. In the other direction, it may be the case that the newer battery discharges faster than the old (lead acid batteries take a few hundred cycles to reach full capacity), so the cells undergo deeper discharges."
6. Correct on over-charging or deeper discharges of other cells in the case of a shorted cell anywhere in a series string. However, isn't a shorted cell way more rare than an open or high resistance cell (that's been my experience, BTW)? Even when a cell shorts, the reasoning in my comment in 1., above, may help to keep the good battery in a series string from being too badly damaged.
I'm still wondering if it's not an advantage having 6 volt batteries in series over 12 volt batteries in parallel - if one is astute/picky/experienced enough to know when only one of the batteries in a series string needs replacing because ... why replace all 6 cells (more money) in a series string when only 3 cells (less money) in a series string need replacing?
