If I have a 90ah lithium ion battery pack can I safely discharge all 90ah of it or will the bms not let me? I've read that the bms only let's you use 80 percent of the rated capacity. So is the real usable capacity 80 percent of 90ah which is 72ah?
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1\$\begingroup\$ You can use all 90 Ah of the battery. But you cannot go below the minimum voltage spec of the manufacturer, typically 2.5 Volts minimum. Most engineers will design for minimum 3.0 Volts because there is very little capacity that can be drawn below 3 Volts. Do you know the difference between capacity in Ah (Ampere-hours) and Battery voltage in Volts? From your question, seems like someone might need add extra explanation for a complete answer. \$\endgroup\$– Vince PatronCommented Aug 7, 2017 at 18:53
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\$\begingroup\$ So what you mean to say is if I discharge it up to 3v I will nearly get 90ah of capacity? \$\endgroup\$– user22483Commented Aug 7, 2017 at 19:52
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\$\begingroup\$ What is your "bms" you mention? Does it come with the pack, or what? \$\endgroup\$– Ale..chenskiCommented Aug 7, 2017 at 19:58
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\$\begingroup\$ Usually all of it, unlike lead acid. \$\endgroup\$– user57037Commented Aug 7, 2017 at 20:14
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\$\begingroup\$ If the BMS restricts you to 72Ah, it should be called a 72Ah battery. High quality lithium ion and polymer cells will deliver most of the Ah capacity even at relatively high discharge rates. \$\endgroup\$– user57037Commented Aug 7, 2017 at 20:18
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1 Answer
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I you have a "90 Ah" battery pack, it doesn't necessary mean it has all of it.
First, the battery capacity is rated at a certain discharge current as [it should be] specified by manufacturer. If you discharge it at higher current, the battery won't have all 90 Ah to deliver.
Second, the actual battery capacity varies depending on charge conditions. The "90 Ah" is likely quoted if it is charged to some specific voltage level and charge current. If you charge the battery under different conditions, it may not achieve the "90 Ah" design benchmark.
In most cases the advertised capacity of no-name batteries can be obtained only under very "relaxing" charge conditions (small current over long time), and in most cases this number is highly inflated.
Third, the end of discharge is determined by the BMS. A good BMS will cut the battery off if its voltage drops below certain limit, so the pack will only "deliver" whatever was stored in it.
In sum, no, there is no guarantee that your battery pack will "safely" deliver 100% of 90Ah of advertised capacity.
answered Aug 7, 2017 at 20:08