{"items": [{"author": "unknown", "source_link": "#", "anchor": "unknown", "service": "unknown", "text": "unknown", "timestamp": "-1"}, {"author": "Michael", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106954618922", "anchor": "fb-10100106954618922", "service": "fb", "text": "I wonder what happens to batteries when they are in parallel, and one runs out of power, and the other starts feeding voltage back into the first one. Is this just a process that evens out the charging status of the batteries? Or is this a possible damaging situation for one or both batteries?", "timestamp": "1566072013"}, {"author": "Jeff Kaufman", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106954618922&reply_comment_id=10100106956205742", "anchor": "fb-10100106954618922_10100106956205742", "service": "fb", "text": "→ If the batteries have the same chemistry (in this case, two sealed agm lead-acid batteries from the same manufacturer) then they should discharge at the same rate in a configuration like this.
If you hook two same-chemistry batteries together, then they'll equalize their charges, one charging the other, and end up equally charged.
If you hook two different-chemistry batteries together you could potentially have a problem, since it's only 12v nominal.", "timestamp": "1566072397"}, {"author": "Gavin", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106954618922&reply_comment_id=10100106960028082", "anchor": "fb-10100106954618922_10100106960028082", "service": "fb", "text": "→ On sailing yachts, this problem comes up practically quite a bit. I'm sure the same on RVs. Everything Jeff said is correct.
In practice, it's a really bad idea to drain batteries in parallel that have not been in parallel their entire life; batteries have limited life, and this is just a dumb way to reduce it.
There's a lot of mechanisms in practice to avoid this; e.g. charge controllers that join the batteries in charging, but never in discharge; and a lot of discussion of relays vs diodies, blah blah blah.", "timestamp": "1566075656"}, {"author": "Gavin", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106954618922&reply_comment_id=10100106960247642", "anchor": "fb-10100106954618922_10100106960247642", "service": "fb", "text": "→ (elide long discussion of spark free make-before-break battery selection switches and voltage controlled regulator operation on internal combustion engines....)", "timestamp": "1566075787"}, {"author": "Gavin", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106954618922&reply_comment_id=10100106960552032", "anchor": "fb-10100106954618922_10100106960552032", "service": "fb", "text": "→ One last point, two batteries of the same chemistry of the same vintage that have been run in parallel in all uses at all times, last quite well. Typical practical experience is that they last quite similarly to a single such battery, but have twice the capacity.", "timestamp": "1566076036"}, {"author": "Greg", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106954618922&reply_comment_id=10100107173904472", "anchor": "fb-10100106954618922_10100107173904472", "service": "fb", "text": "→ Connecting two cells of the same nominal voltage and very similar capacity is safe for discharging, but I would not recommend charging them in parallel unless you build a regulator or use a BMS (battery management system) otherwise any slight capacity differences even just due to temperature variation could lead to accidental over-voltage. I'm not sure if there are decent BMS available for agm, I've only built lithium-ion packs.", "timestamp": "1566231237"}, {"author": "Jeff Kaufman", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106954618922&reply_comment_id=10100107182247752", "anchor": "fb-10100106954618922_10100107182247752", "service": "fb", "text": "→ Greg if you're charging in parallel, both batteries will always get the same voltage, no? So how can one get overcharged?", "timestamp": "1566235564"}, {"author": "Greg", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106954618922&reply_comment_id=10100107183145952", "anchor": "fb-10100106954618922_10100107183145952", "service": "fb", "text": "→ Ah sorry, my mistake, the concern is not over-volting, rather charging one cell faster than the other, which in high parallel batteries is very inefficient... and potentially dangerous if you are charging at high amperage (but that isn't likely to be a concern with AGM which can handle a fairly high charge amperage, or the likely low-amperage charging rates you would be using with e.g. a battery tender).
If you don't use a BMS then make sure to connect the charger such that the positive and negative ends are not on the same battery, this naturally balances, though not perfectly.", "timestamp": "1566236033"}, {"author": "Peter", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106956255642", "anchor": "fb-10100106956255642", "service": "fb", "text": "You should try borrowing an oscilloscope to really measure the peaks. There are handheld LCD ones now that are very convenient.", "timestamp": "1566072464"}, {"author": "Jeff Kaufman", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106956255642&reply_comment_id=10100106956604942", "anchor": "fb-10100106956255642_10100106956604942", "service": "fb", "text": "→ I thought oscilloscopes measured voltage, not current?", "timestamp": "1566072633"}, {"author": "Michael", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106956255642&reply_comment_id=10100106958775592", "anchor": "fb-10100106956255642_10100106958775592", "service": "fb", "text": "→ Jeff Kaufman you would set up a resistance bridge and that would allow you to convert the voltage readings into current.", "timestamp": "1566074603"}, {"author": "Evan", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106956255642&reply_comment_id=10100106959204732", "anchor": "fb-10100106956255642_10100106959204732", "service": "fb", "text": "→ Depending on the details of your wire gauge and oscilloscope, you may be able to simply measure the voltage drop across your ground return cable.
(Be cautious doing this with your scope powered from the same inverter; it may go very badly. Scope probe ground is often connected to scope safety ground, which your inverter might connect to battery negative.)", "timestamp": "1566074939"}, {"author": "Jeff Kaufman", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106956255642&reply_comment_id=10100106959424292", "anchor": "fb-10100106956255642_10100106959424292", "service": "fb", "text": "→ Oh! I already have a resistance bridge (0.5%) as part of my setup, which is how my existing meter is measuring current.", "timestamp": "1566075125"}, {"author": "Danner", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106956255642&reply_comment_id=10100107031010832", "anchor": "fb-10100106956255642_10100107031010832", "service": "fb", "text": "→ I've got a current meter you can hook up if you want. It over that can measure low current draws.", "timestamp": "1566135238"}, {"author": "Peter", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106956255642&reply_comment_id=10100107047852082", "anchor": "fb-10100106956255642_10100107047852082", "service": "fb", "text": "→ Danner You missed the point above. Jeff has a way to measure average current; he needs to measure instantaneous current.", "timestamp": "1566144214"}, {"author": "Jeff Kaufman", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106956255642&reply_comment_id=10100107052188392", "anchor": "fb-10100106956255642_10100107052188392", "service": "fb", "text": "→ Peter peak would also be fine", "timestamp": "1566147053"}, {"author": "Danner", "source_link": "https://www.facebook.com/jefftk/posts/10100106953536092?comment_id=10100106956255642&reply_comment_id=10100107055247262", "anchor": "fb-10100106956255642_10100107055247262", "service": "fb", "text": "→ Peter, my meter is for measuring car ignition current, it has oscilloscope outputs. You can also measure the relative compression of an engine by hooking it up to the starter and seeing the amp draw per cylinder. Are you sure I'm missing the point?
ESI 695 80 Amps DC/AC Low Current Probe https://www.amazon.com/dp/B000FN4IUK/
I suppose I'm missing the point because it would augment an oscilloscope, not replace it.", "timestamp": "1566149488"}]}