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hi youtubers jeff cote here with ask pys so we've got a question from william uh william asks the following he says um this is a good one for all of us actually it correlates battery voltage with battery capacity so just bear with me as we go down this little journey together let's call it a rabbit hole so this boater has a house battery he's a sail boater he's got a 36 foot sailboat catalina like mine but at the end of the day this applies to all of us so he's got a battery bank of two 4d agm batteries that are each about 200 amp hours makes sense so it's a 400 amp hour house battery bank okay pretty standard it could be four golf cart batteries most of us are going to have a battery bank of that size or bigger so at 8 pm he's got 400 amp hours and his battery capacity is 100 and he's at 13.1 volts so basically it just means that he's just disconnected a charger and it's called a surface voltage a 12 volt battery can't be 13 volts obviously otherwise it'd be called a 13 volt battery so it's just going to creep down as soon as loads are added the battery voltage is going to start going down to about 12 6 to 12 8 okay and everything's 100 so overnight he uses about 40 amp hours um so in the morning he wakes up and with only 40 amp hours of battery use so he went from 400 amp hours right down to only 360. so that's not even 10 discharged he notices that his battery voltage is at 11 4 11 7 and he's even seen 10.5 all right now he asks should my battery monitor be seeing about twelve eight twelve six twelve five yeah he's right if you deplete your battery bank only ten percent and you're not running large loads and you slowly discharge your battery bank and this is important you slowly discharge your battery bank like he did overnight 40 amp hours overnight is basically like walking after walking a mile his battery bag that can go 400 amp hours and he's only used 40 so he's only use a 10th of the battery bank should never be seeing 11.4 11.7 or certainly not 10.5 under no big loads so what does that tell us is there a problem with the battery monitor or problem with the batteries it's a problem with the batteries the batteries are effectively what he's found out by putting a battery monitor on his boat is that his batteries aren't giving him what he thought it would and it's time to change the batteries either that or he's got a bad crimp somewhere uh but most likely it's the batteries so sorry to share the bad news william but you're probably gonna have to replace your batteries thank you for watching this video if you have any questions please ask it below or use our contact form on our website and don't forget to subscribe so that you can keep having these weekly and we're producing almost four videos a week now and thanks again for watching