Battery charger question

[Richard19068]

This has probably been asked and answered somewhere but I can't find it. I know that a battery accepts progressively fewer amps as it gets closer to being fully charged but I can't find anything that shows a chart or curve of how many amps it will accept at a given percentage of charge. Either in amps or a percentage of the amp hour rating of the battery. In other words, something like..at 70% charge a battery will accept x percent of its amp hour rating, at 80% charge a battery will accept y percent of its amp hour rating, at 90% charge a battery will accept x percent of its amp hour rating etc. etc. etc...Also, at what percent of discharge does a battery start to incur permanent damage if left at that state of charge? My questions stem from wanting to know how big a charger I really need. I've read several things that imply you should have a charger with 10 to 20 percent of the amp hour capacity of the battery but they never really say what that is based on.I'm asking about shore chargers and lead acid batteries. Specifically, the dual purpose deep cycle ones you get at places like Costco. Thanks,Richard

 

[John S]

Nigel Calder's Books

The questions you are asking are good questions and they are answered in Calder's books about boat maintenance.You will find that the older style chargers (ferro something type) fall off in amps sharply as the voltage of the charged battery rises. Those kind of chargers will also "cook" your battery over time. They are very cheap.One good source of marine grade chargers is the kind that bass fisherman use on their boats (and I am assuming you are talking about AC voltage chargers). The newer chargers for those type of boats have multiple stages, maintain output throughout the rise in voltage and are reasonably priced. Check out Minnkota's website.All batterys have a limited number of discharge cycles. Batterys discharged to half their capacity (notice I am not talking about a voltage value) will eventually fail much faster than if used to only 75% of the amp hours available. No lead acid battery likes to be left discharged. Nicads don't care at all and are shorted out during storage. Trojan battery's website has a lot of information about battery life, discharge cycles, and battry construction.The newer charger types are adjusted for battery type. Bulk, float, topping and conditioning cycles specific to your battery type are available in shore chargers now and will extend battery life over the older type of chargers with their single charge rate. Check it out! John S

 

[jimq26]

Here's two links for you to read.

http://www.batteryfaq.org/

 

[MoonSailer]

Volts and resistence

As the battery is charged it's resistence increases. As the resistence increases you have less amps. I bought a 10 amp charger for 2 100 amp batteries. My alternator takes care of most of the bulk charging as it is 45 amps and then the 10 amps gets the batteries to 100% and then goes to float mode. When I am at the boat sleeping I just leave the charger on. I had a 20 amp charger but it would go at 20 amps for only a few minutes and then go at 2-5 amps for another hour or two so I figured the smaller charger would work for me..

 

[Benny]

There is no quick way to recharge batteries,

safely that is. Try to charge a battery to fast and you will cook it. Nowdays we have the option of some very good multi-stage chargers. They will usually operate three stages, 1) bulk rate or charger amps rated from 0 to approximately 75% of charge, 2) maintenace charge maintaining voltage but lowering charging current to around 3-5 amps/hour from 76% to 98% charge and 3) float stage lowering voltage to around 13.5V and charging current to around 1 amp/hour.Looking at the bulk charge rate of a 20 amp charger and a 10 amp charger it would be easy to infer that the first is twice as fast as the latter but not in real life. First you are not supposed to discharge a deep cycle battery below 50% of its capacity and second there may be many occasions when you do not reach the 25% threshold. In these situations the advantage of the larger charger could be reduced to minutes instead of hours. Remember you only have to recharge the number of amps that you consumed and these come off the top where the charge is slower.Unless you just daysail I would not consider a dual purpose battery. They have a designed shorter battery life. A starter battery is designed to operate at a minimum of 95% charge and if discharged deeply they have between 10-20 cycles in their life. On the other hand a deep cycle battery is design to operate at a minimum of 50% charge and will have around 300-400 cycles in its life. You do the math for the dual purpose but you either get a reduced amount of usable amps per cycle or a shorter cycle life. A lot of sailors accomodate a number of power needs with a two bank system. One bank composed of a single dedicated starting battery and a second bank composed of two deep cycle batteries for house use. When sizing the electrical system start with your usage and then work your way to the batteries and finally to the recharging options.

 

[camaraderie]

assuming wet cells..

You can't even bulk charge at more than about 20% of capacity so it is useless to pay for a bigger charger. Just pick a 3 stage charger that delivers between 10 and 20% of capacity in amphours. If you were to switch to use AGM's...they can bulk charge at up to 100% of capacity on the good ones so getting something more pwerful makes sense if your DOCK time is short. If you leave he boat plugged in all week it really doesn't matter.

 

[Richard19068]

thanks

Thanks for the responses guys. Particularly a link in one of jimq26's links had some interesting info. Not exactly what I'm looking for but helpful.Thanks again.Richard