JHZR2
Staff member
Originally Posted By: GreeCguy
Originally Posted By: JHZR2
You can't play the games you are with paralleled batteries. Both must remain at the same state of charge, depleted the same way, and retain a good impedance match. This is essential so that you don't put one into reversal or create other unsafe scenarios which could cause a hydrogen vent and explosion.
So are you saying the weaker battery could drain the other? Seriously, help me understand. I have a bunch of old batteries from different vehicles and different machines. They are different ages, etc. Would one end up killing the other?
It's complex. You have two things going on - impedance and capacity. Impedance determines the voltage drop under load. A higher impedance battery (which is a function of cell size and design, age, chemistry degradation, etc) will see a higher voltage drop for a given current demand compared to a lower impedance battery. Thus if load is "shared", one will have to put out less current than the other, so voltages stay matched. As you deplete the two, one battery can go into reversal, so the cell goes from +2V to 0V to negative, and then the fun really starts. One battery can also charge the other, current can flow from one into the other at too high of a rate, causing overheating, even fires. Capacity is also related here. Batteries age automatically as a fact that they are a chemical system. So from the time of manufacture, batteries are degrading, with impedance rising and capacity dropping. The low capacity battery is a liability as it will drop and reverse, or else have the issues above. I've seen the effects of mismatched batteries in applications as benign as UPS for computers. Lead acid batteries can and will burn. You don't have sophisticated enough equipment to match and charge them properly.
It may work in practice for a bit, but you'll be damaging the batteries, and putting yourself at risk.
Originally Posted By: JHZR2
You can't play the games you are with paralleled batteries. Both must remain at the same state of charge, depleted the same way, and retain a good impedance match. This is essential so that you don't put one into reversal or create other unsafe scenarios which could cause a hydrogen vent and explosion.
So are you saying the weaker battery could drain the other? Seriously, help me understand. I have a bunch of old batteries from different vehicles and different machines. They are different ages, etc. Would one end up killing the other?
It's complex. You have two things going on - impedance and capacity. Impedance determines the voltage drop under load. A higher impedance battery (which is a function of cell size and design, age, chemistry degradation, etc) will see a higher voltage drop for a given current demand compared to a lower impedance battery. Thus if load is "shared", one will have to put out less current than the other, so voltages stay matched. As you deplete the two, one battery can go into reversal, so the cell goes from +2V to 0V to negative, and then the fun really starts. One battery can also charge the other, current can flow from one into the other at too high of a rate, causing overheating, even fires. Capacity is also related here. Batteries age automatically as a fact that they are a chemical system. So from the time of manufacture, batteries are degrading, with impedance rising and capacity dropping. The low capacity battery is a liability as it will drop and reverse, or else have the issues above. I've seen the effects of mismatched batteries in applications as benign as UPS for computers. Lead acid batteries can and will burn. You don't have sophisticated enough equipment to match and charge them properly.
It may work in practice for a bit, but you'll be damaging the batteries, and putting yourself at risk.