Slipperman
Well-Known Member
Are sealed lead acid batteries as long lasting as the ones you have to top up from time to time? Or am I right in thinking that if you never have to top them up, they cannot possibly last as long?
Sealed Lead Acid Batteries
- Thread starter Slipperman
- Start date
Rigger Mortice
N/A
In my experience there's little difference....with one proviso. If you overcharge a sealed battery and let it boil then, if you can't top it up, you are likely to have problems.
In any event most batteries seem to be sealed these days.
I don't know if its different with real deep cycle batteries. I've never had one.
In any event most batteries seem to be sealed these days.
I don't know if its different with real deep cycle batteries. I've never had one.
VicS
Well-Known Member
Some relevant info here: http://batteryuniversity.com/learn/article/lead_based_batteries
pvb
Well-Known Member
I guess you're talking about "maintenance free" batteries. Technically, they're not "sealed", but you can't top them up. They tend to use lead/calcium plates which help make them more resistant to overcharging. They also have a fairly large reserve of electrolyte, to compensate for the fact that they can't be topped up.
Used sensibly, they can last a long time. I've used them for my domestic bank and start battery for years. My boat has an Adverc alternator booster, so charge voltage can be 14.8v much of the time. The last lot of batteries I had lasted 8 or 9 years, which I don't think is bad.
pvb
Well-Known Member
Vic, I'm not sure that link is entirely appropriate. It claims that maintenance-free batteries are "acid starved", and says that VRLA batteries are gel batteries. This might confuse people. Today, most common-or-garden maintenance-free batteries are basically flooded lead acid.
Plevier
Well-Known Member
I was going to make the same point. "Sealed maintenance free" and "sealed lead acid" (also known as VRLA and AGM), technically completely different, are often confused.
Worth noting that "sealed maintenance free" very often can be topped up - frequently you can find the fillers just by peeling off the label on top! They won't necessarily be screw plugs, often a strip to lever out.
Last edited:
Rigger Mortice
N/A
pvb
Well-Known Member
They were Delco 2000 batteries. I replaced them with Merlin PowerPack batteries, which are similar in construction to the Delco batteries.
RAI
Well-Known Member
One disadvantage of sealed batteries is that you cannot use a hydrometer on them, which is a very simple way of detecting a dying cell.
Slipperman
Well-Known Member
Thanks for replies so far. I was aware that some so called 'maintenance free' batteries can actually be topped up but am attracted to the totally sealed type because with my present batteries (which can be topped up) I find that I do get a small amount of leakage at high angles of heel. Just wondered if they would last as long.
One other query raised above - how would I be likely to make my batteries boil? I don't have any special battery monitoring system, but can I assume that charging from the engine or via a battery charger fitted as part of a properly installed shore power system would prevent such boiling? And how do gases vent from a totally sealed battery? Sorry, that's two more questions.....
One other query raised above - how would I be likely to make my batteries boil? I don't have any special battery monitoring system, but can I assume that charging from the engine or via a battery charger fitted as part of a properly installed shore power system would prevent such boiling? And how do gases vent from a totally sealed battery? Sorry, that's two more questions.....
VicS
Well-Known Member
I understand that totally sealed batteries have system of recombining the hydrogen and oxygen produced... at least in normal quantities ... No idea how it works though. The batteries are maintained a slight pressure by a valve which will vent off excess gas if the need arises.
A properly regulated alternator or charger should not boil the battery or even produce excess gassing.
Plevier
Well-Known Member
Crudely speaking the battery is configured so that nascent oxygen produced at the positive plate migrates through to the negative plate where it recombines to give water and heat. If you really want more detail I can dig it out but it's no longer at the front of my mind! (And I'm an elec engineer not a chemist.)
In gel batteries it doesn't occur when the battery is new. Initially a gel battery will gas until the electrolyte has dried out enough to develop micro cracks that provide the transport medium.
An AGM battery is produced with the microporous absorbent mat deliberately unsaturated and micro-voids in it provide the transport medium. Again they may be made a bit over-wet and a little bit gassed off in the initial charging.
Transport in the AGM separator is much more efficient which is why you can charge them at higher rates than gel. In either case if you charge too fast/too high a voltage you will get gas liberated at the plates - and water loss - because you overload the transport capability. Free gas produced does not get recombined, it vents. Catalysts have been tried to recombine free gas but not very successfully and rather expensively. A gel or AGM VRLA battery does not include a catalyst, contrary to common belief.
They are called starved electrolyte because whereas flooded cells generally have a gross surplus, it's a difficult design balance in an AGM to hold enough and if you deep discharge it the elctrolyte will become as near as dammit water, which is why you can need a very high voltage to start the recharge (but you must back it off very quickly when charging starts). For the same reason the charged specific gravity tends to be higher, up to 1300 against the normal 1260.
Again to ease the transport you have to have quite thin plates and close plate spacing and that is a big factor in the normally very good high rate capability.
In gel batteries it doesn't occur when the battery is new. Initially a gel battery will gas until the electrolyte has dried out enough to develop micro cracks that provide the transport medium.
An AGM battery is produced with the microporous absorbent mat deliberately unsaturated and micro-voids in it provide the transport medium. Again they may be made a bit over-wet and a little bit gassed off in the initial charging.
Transport in the AGM separator is much more efficient which is why you can charge them at higher rates than gel. In either case if you charge too fast/too high a voltage you will get gas liberated at the plates - and water loss - because you overload the transport capability. Free gas produced does not get recombined, it vents. Catalysts have been tried to recombine free gas but not very successfully and rather expensively. A gel or AGM VRLA battery does not include a catalyst, contrary to common belief.
They are called starved electrolyte because whereas flooded cells generally have a gross surplus, it's a difficult design balance in an AGM to hold enough and if you deep discharge it the elctrolyte will become as near as dammit water, which is why you can need a very high voltage to start the recharge (but you must back it off very quickly when charging starts). For the same reason the charged specific gravity tends to be higher, up to 1300 against the normal 1260.
Again to ease the transport you have to have quite thin plates and close plate spacing and that is a big factor in the normally very good high rate capability.
Last edited: