leisure batteries for motorhome / rv

Hi , I am going to seem a right dummie to some of you out there , but have been trying to get to the bottom of this for while and hope that you experts out there can give me the answers. I want to understand the releationship between amps storeage and volts. I understand the principal of watts / volts = amps consumption per hour etc. The reason being , I have a Motorhome , you guys accross the water call them RVs that has 2 x 110 amp acid leisure batteries in parallel. Fully charged they show 14.5v gradually dropping back to 13.0v when the mains charger has been disconnected for a while. After consuming what I work out to be approx 45 amp in two days , the TV picture cuts out and the inverter sounds the warning signal and the meter is reading 11.0v. Turn off everything and the meter jumps back to 12.5v. I have checked the meter and all is fine there so I know it is accurate. The batteries are only 12 months old , nothing to say about them other than a couple of weeks ago I inadvertantly left the internal lights on whilst laid up for a few days and ran them down to 10.5v. Dont know if this caused any damage or not? Can someone help me understand what is happening here so I can determine if I have a battery problem or not. Thanks in advance guys.

Batteries usually charge at up to 14.4V. They are usually 12.67V fully charged. Your reading of 13V and above is "surface charge" which can take up to a day to dissipate. Usually after stopping charging at (say) 14.4V, the battery drops quickly to around 13.5-13.7V, and eventually to ~12.7V. A quick drop to 12.7V may indicate aged batteries. eg a high internal resistance that discharges the surface charge quicker, or a general lack of capacity or collapsed cells etc. FYI - If using voltage to measure battery condition after charging, for new car batteries, 10 or more minutes with headlights on may be recommended to remove surface charge, else leave disconnected (ie, OC = Open Circuit) for 24 hours. A fully charged battery is 12.6 - 12.7V OC (no load). (In theory it's 12.67V @ 25 degrees C.) A fully discharged battery usually about 1V less, but can vary from ~11.3V to 11.8V.     For cranking batteries, the usual recommendation is not to discharge more than 20% - ie, ~0.2V down from its full voltage, so maybe down to ~12.5V or ~12.3V. For deep-cycle batteries, the recommendation is no more that 50% discharge, so maybe down to ~12.2V or ~11.9V depending on the battery. (The same recommendation applies to deep-cycles that specify 80% discharge.) BTW - the OC voltage drop is linear between its fully charged and fully flat voltages wrt its remaining capacity. OC voltages are not to be confused with "loaded" voltages. Internal resistance means a voltage drop at the terminals, and the internal resistance increases as the battery discharges. Hence it is common to see battery a "end voltage" specified at lower a voltage - eg, 10.7V @ 20A, or 11.3V @ 10A, but that might correspond to an OC voltage of 12.0V (remove the load and measure). Batteries will also self-recover after removing loads (unless they are in really bad condition). I'd suggest your batteries were damaged after their flattening. The cells probably haven't collapsed since it recovered to 12.5V, but they may be heavily sulfated. But they will require battery maintenance - ie, boost and equalisation charging above the normal maximum of 14.4V, and probably using 20A or more. The sooner the better because the sulfation that forms when not fully charged goes hard after about a week and is then impossible else difficult to remove - the loss of capacity is then essentially permanent. Though batteries can often recover from over-discharge (flattening), the older they are, the less they recover. And any battery should be recharged ASAP after a discharge. You could have a look at Bill Darden's batteryfaq.org - namely the top link to Car and Deep Cycle Battery Frequently Asked Questions (FAQ) (or even download its zip file as per instructions under the last section link Battery.Zip). Also, I hope you are disconnecting your batteries from each other when not in use. (Or as often as possible when not required or not being charged.) If one is worse than the other (a lower voltage after recharging), do not reconnect them to each other unless you're milking the last out of them - the bad one will damage or destroy the good or better one. And if you replace one, replace both!

Hi , many thanks for the info and your time. I have found out a bit more about the batteries that I have. They are Absorbed Glass Matt type and I am told that Equalisation is not possible ?? I have been told to cycle them on a charger outside of the motorhome to try and build up the amps again ??? It is taking a long time as I only have 4 amp car battery charger. I do another 4 amp charge but i dont know if it is possible to use two at once , would that make it an 8 amp charge? Do you anything about this type of battery and what would be your advice to try ad revive them? Thanks

I thought I included my usual AGM rant. Rats! Alas the problem with AGMs, wrt over charge and over discharge they are far less tolerant than wet cells. AGMs are also known as VRLA but NOT to be confused with gel cells which are quite different in some respects. AGMs are difficult if not unlikely to recover, hence the need to heed their "do not over discharge" warnings, and their "recharge ASAP" or immediately after certain (deep) discharges. But some AGMs like the Ca/Ca types can be equalised in accordance with the manufacturer's instructions. See 9. HOW DO I CHARGE (OR EQUALIZE) MY BATTERY? in that BatteryFAQ zip or link. Generally recovery requires above normal charge voltages (at high current) else above normal currents. But both are generally not possible with AGMs because of the risk of explosion etc or simply that that process damages the plates or plate/matt interface (eg, bubbles). One option might be a suitable pulse charger - ie, "intelligent" or micro-processor controlled, not the old "dumb" chargers. Otherwise maybe you could try putting it in the car and letting it charge (after a jump start?) since the alternator can output its spare capacity (at whatever RPM) provided it does not exceed 14.4V with no collapsed cells. But as per Section 9 in BatteryFAQ, that could mean an over-current for that AGM (if the battery accepts all the alternator's available current). And not that I am recommending or endorsing nor suggesting that further damage or explosion will NOT occur, but maybe if such over-current is for a short duration only (1 minute or 2??), then if any recovery occurs, maybe it will outweigh the damage. Maybe. But that's only a last ditch experiment if all else fails. You'll have to decide how much it's worth to give the batteries to a suitably equipped knowledgeable professional to try the recovery as opposed to buying a new pair. As to paralleling two 4A chargers, that firstly depends on the chargers. If they are the old dumb transformer types without a uPC, then it probably won't damage the chargers. But make sure you don't exceed 14.4V at the battery terminals - and that assumes there has been no internal battery cell collapse. (If there has been, that means gassing and a reasonably likely explosion or thermal runaway!) But secondly, might it do more damage? If 8A is under the battery's recommended charge rate, then ok. But really they should be given to "the expert" ASAP. And note that I am not an expert per say wrt reconditioning damaged AGMs. Should anyone with qualified experience suggest - or Bill Darden write - differently, then they overrule and supersede my advice.    But I strongly suggest finding a suitable expert, else trying what your advisor advised, or buying a new pair. Don't consider me "responsible" for any suggestions above - ie, forget any notion of blaming me or suing me if nothing or the worst happens. I also suggest you consider a battery protector aka low-voltage cut-out to protect against similar damage in the future. EG - the MW728 is about $20 and cuts out below 11.2V and in above 12.5V but has no protection against "cycling" due to battery self recovery, and it draws 10mA (from memory) when merely monitoring the battery voltage (ie, 10mA standby current). Its 10A switching limit can be increased by using it to turn n a bigger relay. But you might want something more sophisticated.

Just to add to the above about not being able to recondition AGM batteries, the November 2012 edition of the Australian "Silicon Chip" magazine (p96) in response to a reader complaining about Silicon Chip's October 2009 "Battery Zapper/Rejuvenator Kit" not rejuvenating his SLA batteries - "... sealed lead acid (SLA) batteries cannot be rejuvenated. If they have discharged below 11V, that generally means death.". I'll merely add that by SLA they mean AGM and probably gel-cels (there are flooded types occasionally {mis?}labeled as SLA), and that the 11V should refer to their open-circuit voltage (as opposed to their loaded end voltage).