March 27, 20197 yr I was wondering if it's possible/advisable to run 2 battery packs in parallel to increase the range? As long as the batteries are the same voltage and similar capacity I don't see a problem. Look forward to your feedback.
March 27, 20197 yr They don't need to be the same capacity. Ideally, they should be of the same basic chemistry so that they have the same fully-charged and fully-discharged voltages. Two important rules: They must be approximately at the same measured voltage when you connect them; and you must disconnect them from each other when you charge them.
March 27, 20197 yr Author When you say same basic chemistry, could you mix lithium ion and lithium polymer cells?
March 27, 20197 yr When you say same basic chemistry, could you mix lithium ion and lithium polymer cells? Putting power sources in parallel is electrical engineering 101 . If both sources are initially discharged then their internal resistances are high and they can be safely connected in parallel. If both batteries are both charged and have IDENTICAL voltages, they can be joined in parallel . ..even if their internal resistance is tiny. However if there is any difference ,external resistance must be added... assume that one battery was at 40.1 and the other 42.8 , the difference is only 2.7 v ,but if the internal resistance were say 10 milliohm , then the current would be 270 amps. It is normal in a professional setting to use a set of lamp bulbs of different resistances and voltages connected between the two batteries and as each of the larger voltage bulbs extguishes , more sensitive bulbs are switched in. Now if the chemistry of the batteries were different, or even cells at different temperatures, then there different terminal Voltages Edited March 27, 20197 yr by Danidl
March 27, 20197 yr I'd say no because they will charge to different voltages and have different discharge characteristics. I've been running two batteries for a while now but they are both Li-ion. I measure the voltage at the terminals EVERY TIME before I connect them together. Sods law says that the time you don't do it will be the time one hasn't charged fully. I've connected them with 0.3v difference with no ill effect. I started to get nervous when the difference was drifting to 0.4-0.45 and so the older battery is currently being re-celled. Folk here have said 0.5v should be fine - didn't want to push it.
March 27, 20197 yr I have and do use lipo and lion in parallel and both work fine together and find discharge to be equal. Once I have a resting voltage of 3.6/3.7v min per cell I disconnect the lipo and carry on with the lion, my lipo's make discharge easier as I have added a bms with switch to them to help regulate discharge. I rewired the graphene's 6 & 4s as 10s lipo pack's with 20a discharge bms. I connect batteries within 0.03v. Edited March 27, 20197 yr by Nealh
March 27, 20197 yr When you say same basic chemistry, could you mix lithium ion and lithium polymer cells? Yes, that's OK. I mean more like Li-ion and LiFePo4 or lithium titanate. It's not that they don't work together, but it's more that you get unusual discharge curves that can catch you out.
March 27, 20197 yr Putting power sources in parallel is electrical engineering 101 . If both sources are initially discharged then their internal resistances are high and they can be safely connected in parallel. If both batteries are both charged and have IDENTICAL voltages, they can be joined in parallel . ..even if their internal resistance is tiny. However if there is any difference ,external resistance must be added... assume that one battery was at 40.1 and the other 42.8 , the difference is only 2.7 v ,but if the internal resistance were say 10 milliohm , then the current would be 270 amps. It is normal in a professional setting to use a set of lamp bulbs of different resistances and voltages connected between the two batteries and as each of the larger voltage bulbs extguishes , more sensitive bulbs are switched in. Now if the chemistry of the batteries were different, or even cells at different temperatures, then there different terminal Voltages I agree with everything but the last sentence. if two batteries are connected in parallel, you can't have different voltages on the terminals.
March 27, 20197 yr To a point lipo and lion discharge fairly uniformly down to about the 3.6v per cell, generally with lipo discharging deeper doesn't gain you much in range so I tend nowadays not to go lower.
March 27, 20197 yr I agree with everything but the last sentence. if two batteries are connected in parallel, you can't have different voltages on the terminals. Unfortunately you can and then you have large equalization currents so that the voltage in the loom can remain the same . The concept that all points on a wire are at the same voltage is a circuit theory idealised situation If the cell technology is different and the full charge voltage state different, the combined pack will charge to the lowest full value and even if the other battery pack had extra capacity it never gets used.
March 27, 20197 yr Unfortunately you can and then you have large equalization currents so that the voltage in the loom can remain the same . The concept that all points on a wire are at the same voltage is a circuit theory idealised situation If the cell technology is different and the full charge voltage state different, the combined pack will charge to the lowest full value and even if the other battery pack had extra capacity it never gets used. It sounds like you've never put batteries in parallel yourself. It's absolutely impossible for two thick conductors joined together to have a voltage difference at the joint. You have to separate them to get a different voltage in each.
March 28, 20197 yr It sounds like you've never put batteries in parallel yourself. It's absolutely impossible for two thick conductors joined together to have a voltage difference at the joint. You have to separate them to get a different voltage in each. No... Read what I have written. ..all of it. The voltage on the line should be the same at every point ,but will be different until the current in the bussbar drops to zero. ... it is not absolutely impossible ,it is in fact always the case. The slightly higher voltage device will drive current into any external load, and also drive current into the slightly lower voltage device. With low resistance busbars and low resistance batteries, the interbattery current can be enormous. Eventually the slightly higher voltage gets depleted a bit, its internal resistances rises a little and the unbalanced current drops towards zero. The current taken from the two batteries will be inversely proportional to their internal resistances.
March 28, 20197 yr I also use LiPo and Li-Ion in parallel - it brought new life to my tired bottle battery.
March 28, 20197 yr Hi. I wired and mounted a dual 40 amp (20 + 20) Schottky Diode on a square piece of aluminium and stuck it in a tic tac box, as my bike battery is still under guarantee and has a smart BMS, and there is an instruction not to ever send reverse current into discharge port which can happen due to battery voltage differences and also because of a sort of parasitic current due to differences of battery internal resistances, which can occur during rest period after heavy discharge (e.g a hill climb). There is a small sacrifice of about half a volt for this but this is easily made up by the fact that the twin batteries has much less sag and there is no need to worry about battery voltage inbalance. https://www.ebay.ie/itm/Vishay-MBR4045PT-E3-45-Dual-Switching-Schottky-Diode-Common-Cathode-45V-40A/173385936086?hash=item285e9b6cd6:g:v8MAAOSw~jpbNlOW Edited March 28, 20197 yr by Sturmey
March 28, 20197 yr There must be some curious things going on when two slightly different potentials are connected. All I can go on is what I'm capable of measuring. When I measure my individual batteries, before connecting, I may see 41.5v and 41.8. When connected I will see 41.6 or7 at the end of the Y lead (dvm doesn't have a second decimal place display). That's instant, so there must have been some sort of balancing current flow, but at 0.3 or 0.4 volts into the 10 milliohms, suggested by Danidl, I get that to be 4amps. Presumeably that will flow from the higher battery to the lower via the discharge circuits, and quickly diminish as the batteries equalise. I wouldn't want to go above 0.5v difference though. In use, the ensemble shows far less sag and I no longer have to avoid the killer climbs if I don't want to. Currently getting one battery re-celled so they will both have the same cells, though differing ages.
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