Dasterdlydan,
Willib was just offering a possible workaround no need to type in caps, it's considered yelling. If you need to emphasized try using bold type, I believe putting your text between two asterisks when typing does that on this board.
As to your battery voltage question, if I recall correctly you should be looking at somewhere around 8.56 volts or a little above for an 8 volt nominal battery. The real problem you have is that series strings of 8 volt batteries are really only practical in 24 and 48 volt systems. You can string up three for 24 volt and six of them for 48 volt systems respectively. There isn't really a way to properly/advisably configure 8 volt batteries for a 12 volt system.. unless the individual cells are linked with exterior bus bars in which case you could bypass two cells in the string to get to 12 volts nominal. This is a common trick on nicad packs being used with 12 volt inverters. The trade off of course is reduced capacity if those cells are left unused... there are other creative methods around that too... however I do not believe your batteries have bus bars, am I incorrect?
On your other question, unless that 6 volt battery and the 8 volt are alike in every way except the number of cells they have I advise against series strings, The reason for this is that if the individual cell capacities are different, as they most certainly are, the smaller capacity cells will overcharge while the larger ones continue to try charging, effectively frying/cooking/overheating the smaller cells while the others are still charging. The smaller cells would be damaged and the larger would be under charged. This is something that can be quite spectacular during failure or a long process over several charge/discharge cycles.
If however their individual cell capacities are identical than a series arrangement of 6 & 8 volt batteries is fine.
So, yes, if you have cells of identical capacity than both 6 and 8 volt batteries will charge properly when in series, but you have a problem as the nominal voltage is 14 volts in this arrangement... I notice Nando has already told you this bit of information... and he is right! Under charge asumming you would be fully/properly charging the 6-8 volt arrangement you would still trip your inverter into over voltage disconnect.
I conclude you may want to consider a switch to a higher input voltage system/inverter if 8 volt batteries are going to be more common in your supply chain. Alternatively investigate DC Autotransformer as another possible solution to voltage missmatch... it is in fact not really a transformer so much as a bi directional dc to dc converter.
Lastly the short term measure involving the 6-8 volt series question may be as simple as dropping the input by a few volts in a manor similar to what willib sugested to you... but you still have a missmatch... notably with your charging source.
The bottom line is 8 volt batteries and 12 volt systems were not made for each other!
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crashK6
