dmr

'm very very slow at the keyboard! and its a poor internet connection down here in Wiltshire. Woke up this morning with thoughts of voltage vs specific gravity going round in my head, time to pull the pins and do some boating I think. ............Dave

I am very naughty with my equalisation, I do it from the alternator (by adjusting the Adverc voltage) and so like to do it quickly to avoid running the engine for ever. I start equalisation once the charge current is down to about 10amps. When I increase the voltage to 15.5v the current goes up to about 25 amps, though drops down to maybe 15 quite quickly. This slightly brutal equalisation does the job. I do not get massive water loss, the batteries do not get hot, and they are now over three years old,cycled everyday, and still looking good. I do make sure I have good ventilation. Hence I think a 1.5 amp charger is way too small. I Like the idea of a power supply but get a beefy one. Charging pairs of batteries sounds tedious, get a charger/psu than can do the whole lot together (turn off anything in the boat that might struggle with 15.5 volts). The split pairs concept is Petes idea (and maybe mine) but I think its only worth doing if you are not willing or able to run an engine or generator and so need to use a "battery to battery" type charger. ...............Dave

I reckon Nick is totally wrong here. Not very often that happens. Even when he's just making it up he's usually right. Two ways to look at this: Two 100Ah batteries, one at 100% and one at 50%, will be the same as a 200Ah battery at 75% charge and the voltage will reflect this (more or less) Other Approach Two voltages sources with a source (internal) resistance. Combined voltage is then somewhere between the two voltages based on "potential divider" made from the two resistances. 100% battery will have Slightly lower resistance, and all the electro-chemistry will add some complications. Some current will flow from 100% battery to the 50% battery till they are balanced, but due to the voltage overhead needed to charge a battery this current will be low and things will happen very slowly. In our boaty life we will discharge the batteries long before they balance and so initially more current/charge will come from the 100% battery. ???????????? ..............Dave

Yes but the Smartgage will enhance your geeky fiddling because you can then actually confirm that you have got the fiddling right. It is expensive and it does lack a bit of style and flashness! It looks like it was produced by an engineer without a marketing department!. There is something very satisfying about a simple direct readout of state of charge that always work. I also like it because if I need to work away Gillie can easily take over with the engine running and state of charge monitoring. I want to get an amp hour counter but fancy making one myself but have not found the time to do it yet. .........Dave

Replies to various above posts... The Smartgage is worth every penny. I believe that stronger acid reduces life expectancy of batteries so manufacturers are likely to add excess water rather than excess acid. I have had a shot at working out how much the SG is reduced by adding extra water, had to make a couple of assumptions. With a SG of 1.270 at the "correct" level, I reckon adding 100ml of water will raise the level by 6.9mm and reduce the SG to about 1.257. Is the terminal voltage reduced by a similar factor?????? ............Dave

Thank you for accepting your error with dignity. I am sometimes wrong but often right. When I'm wrong my wife reminds me that I am wrong. When I'm right she tells me I'm an infuriating smug Bstrd. Your batteries are a bit full and this will reduce your SG a bit so things are even better than they look. I tried to work it out today but I need to know the volume of acid in a Trojan Cell and the www failed to tell me. .................Dave

I do struggle with this, and the various sites on the www do conflict with each other.. Your acid is cold and hence extra dense so will have a higher SG, so I still reckon you need to Subtract the correction factor from your readings. or you need to ADD the correction to the figures provided by Trojan. This is probably the source of the confusion. Lets see what the others say????? ...........Dave

Well done, but I may have to deliver a little bad news. That temperature correction goes the other way. It makes sense, at low temperatures the acid shrinks so is denser, so the fully charged SG should be 1.277 + 0.012. But don't worry about the details, its all a bit approximate and you have massively improved things. As you too are now becoming a boaty battery expert why don't you get a Smartgage? With both a Smartguge and your amp-hour meter you can make some serious measurement of battery capacity and health. ................Dave

I have read that the concrete floor thing is no longer true, and comes from the old days when battery cases were made of some sort of hard rubber or similar. A few years ago I bought a set of batteries from a motor factor and found they were heavily sulphated. The shop returned then to their supplier who initially refused a refund because the batteries were sulphated !!!!!! ................Dave

I agree with you (and about the 49-51 thing, but our obsession with rigid thresholds is one to discuss another day) I've been looking at the www again. (trying to fix obscure software bugs today so keep looking at the forum and www instead). There is some suggestion that battery volts relates directly to specific gravity.. We know that when a battery gets sulphated its SG will change. Also I guess if an old battery looses lead oxide (or lead sulphate) then this could change the SG and hence voltage.. So an old battery may well have a changed voltage for a given state of charge. Like you I don't really think this matters except in extreme cases,, a battery bank may move a bit of charge around to equalise things but I don't envisage any huge currents flowing. Also the initial discharge will come from the strong battery till all is equal. I suppose in extreme cases the good battery will try to pull up the weak one and waste some electricity doing so because the weak one is already full. But if we are going to worry about this then we also need to worry about the normal (significant) cell to cell variations and equalise every day! ..............Dave

Did Gibbo not say that the Smartgage will not work on batteries that have lost more than 50% of their capacity???? So in extreme cases the battery must change more than just becoming a lower capacity battery. I suspect that the internal resistance and/or voltage characterises will change significantly. Still this is all a bit academic as once a battery is down to 50% it really should be going into the bin anyway (ok to the recyclers). ............Dave

The standard method is to measure the specific gravity every hour and to continue till the specific gravity is no longer increasing, or when the batteries get too hot. Several hours in some cases. ..............Dave

Arguing about CaRT rules, continuous cruisers, and continuous moorers can be fun (why don't we do that on this forum anymore?). Arguing about electricity has little potential (!) for debate. Nick is right and that's it.. Most alternators only sense and regulate the voltage at the alternator,nothing more.The individual batteries then consume as much current as they desire. A few measure the voltage of the combined battery bank. None can look at the voltage of the weakest battery. ...............Dave

A voltmeter in the bedroom, that's serious stuff. Are you worrying about your batteries a bit too much?. We sleep in the backcabin so I can just about see the ammeter in the engine room from my bed, but I have to sit up to get a line of sight to the Smartgage.!!!!! ...............Dave

Daily Backups? that's a bit like giving up beer; a very good idea that just isn't going to happen. I work on the concept of "phew, got away with it again". Thanks, I have never heard of Everything, I am investigating now. I doubt if Firefox has actually lost my bookmarks, I just can't find them after the latest user interface change. Its something that I get irritated about, I see computers as tools and want to get on with my jobs rather than learning my way around a new user interface every few weeks. I write software and do sometimes have to make user interface changes and I really worry about it as I know some users will be frustrated about it. I could never justify it for "fashion" reasons. rant over.......back to specific gravity. .............Dave.

Can you find a date stamp somewhere on the batteries? If so you should find how to "decode" it somewhere on the www.. Batteries do discharge themselves over time but this is very temperature dependant. As they may have a hint of sulphation I would charge them at at least 14.8 volts, and maybe do an initial 15.5 volt equalisation charge. I would also check the specific gravity with a hydrometer. ...........Dave

Alternative executive summary.... Best not to mix old and new, but acceptable as long as batteries are of the same type (chemistry) and as long as you make sure to Never let any batteries get past their "die by" date. A brand new set of identical batteries will show some cell to cell variation so no point in going overboard on "matched sets". ...............Dave

I don't think the capacity should be massively reduced during the first 100 cycles, just a bit. I saw a graph somewhere on the www, I will try to find it again. A recent Firefox update has lost all my bookmarks, these open source geeks are even worse than microsoft. ..............Dave

Yes, just keep an eye on temperature. The cheapo eBay infra-red thermometers have all sorts of uses when you live on a boat (-2 on the well deck right now) Equalisation will most likely give a tiny reduction in battery life, but uncontrolled sulphation will certainly give a massive reduction in battery life. ...........Dave

It sounds like the equalisation is starting to work, but if there really are significant variations from cell to cell then you need to do More. Why do you think you dare not do more?? You wont hurt them. An hour is really not enough if you have a bit of sulphation. oooH your batteries are much too full so do keep an eye on them during the equalisation. As your batteries are over-full then this will lower the SG that you measure. Not sure by how much, I keep meaning to work this out but with a bit of luck Nick will do it???? Another little mystery...when you top up the Trojans the capacity appears to reduce significantly for a couple of cycles! ................Dave Try to avoid any temptation to take acid out of the batteries. Search for a recent thread on that topic. Just go carefully till it sorts itself out. ...........Dave

Matty is right here. Electrically its fine to mix old and new, they will work together and whatever capacities they have will add togeher BUT old batteries will fail at some stage and this might be catastrophic (and dangerous). with the healthy one discharging into the failed one. Trouble is if you mix old and new you will loose track of just how bad the bad ones are, and likely forget just how old they are. So I would say the only time you should mix old and new are.... 1 A relatively new battery fails and needs replacing. 2 Money is tight and you can only afford 1 or 2 this month.....but make sure you replace the others next month. If in doubt replace the lot. ............Dave

Charge to 100%, measure SG with your hydrometer, report back!!!! Trojan are most likely thinking in terms of a golf cart battery that gets put on charge overnight every night for a long time. Boating gives batteries a much harder life and some of what they might not apply. For example,when I spoke to them (with similar problems to you) they suggested charging at 15v rather than 14.8v! My rough sums suggest you are charging to less than 90% and discharging to 30% Get that hydrometer out! ............Dave

The low tail current does not always mean fully charged, as in 450Ah, its just means as full as they can get in their present condition. This is why sulphation is such a bugger, the battery looks just like it is fully charged but its not. Get that hydrometer out again, its the only real way to confirm that equalisation has got the batteries back to full capacity. I know its all messy and a pain, and makes holes in yer clothes, but once you've got this sussed out you will need to use it less and less. The SG of the Trojans should be just as high as the starter, until they are you will be digging yourself into a hole.. We have to get a whole lot ruder, outspoken, (and sometimes very funny) before we can take over from Gibbo. ............Dave

>>> If you start at 100% and take out 187aH, with a light load I would expect about 12.1 volts, so your 11.67 is too low <<< Big problem is that none of us knows how the cold is affecting our results (especially mine) I might try to do something tomorrow. It means turning everything off but maybe I can do that when I walk the dog. As Nick says an amp-hour counter can get things badly wrong and this may well have resulted in some sulphation. (Equalisation will fix this). Lets have a shot at working this out... 11.67 volts. Add 0.1 for your load effect gives 11.77. Thats about 28% charge (ouch) Add your 187aH (58%) and this suggests your batteries started out at 86% rather than 100%. Did you say your specific gravity was just into the green? ( = 1.250? = 87% charge !!!!!!!!) This is all a very rough estimate because you won't get the full 450aH with a cold battery. .............Dave

This is the results of todays experimentation. (sorry its not bigger, will try harder next time) The solid line is the published Trojan Voltage vs Charge. The dots are my measured battery voltage I took state of charge from the Smartgage. I also took battery voltage from the Smartgage. Smartgage is not the worlds best voltmeter,and only displays to 0.05 resolution, and jumps about a fair bit. When it was obviously jumping between two values I estimated an in-between value. The discharge current also jumps about a bit so I made a visual average. Current was mostly 4.5 to 7 amps, though one point was 7.8. The very first point was 0.18 amp discharge before I turned the computer etc on. The last point was 6 amps, but I then increased this to 11 amps for 5 minutes and took another reading. The odd value at 67% is a mystery, I repeated it and got the same reading. Battery temperature was low at 4 to 6 degrees C So I conclude that a light load reduces the terminal voltage by a bit less than 0.1 volts. BUT I have not done any temperature correction. I was not able to go below 63% charge but there is no sign of any divergence over the measured range so I would be surprised if anything dramatic happens at lower charge. ...............Dave