billskip
Well-Known Member
????
Circuit design is the design that is the power consumption...how on earth could you consume power without a circuit?
‘Fessing up. My dubious LiFePo install.
- Thread starter fredrussell
- Start date
Circuit design is the design that is the power consumption...how on earth could you consume power without a circuit?
geem
Well-Known Member
billskip
Well-Known Member
Frightening innit.....
lustyd
Well-Known Member
By the same token, if all we need do to reduce power consumption is add a resistor, electric cars could be infinitely more efficient. All of these things are interlinked, but the one thing that's certainly true is a circuit with warm wires is less efficient than one with cool wires, so all else being equal it's a worse circuit for getting power to the batteries.
noelex
Well-Known Member
All wires have resistance. As current passes the wire, it heats up due to this resistance. This energy has to come from somewhere. The current has to remain constant throughout the wire; therefore, the energy comes from the voltage drop.
When charging a battery, the effect of this voltage drop is to lower the charge voltage. If we knew the distance, we could calculate the voltage drop accurately, but a 6mm wire and the associated connections may well be dropping around 1V with a 30A current over the distances I would expect. Thus, when the alternator was delivering its likely charge voltage of around 14.2V, the battery is only receiving 13.2V. Batteries don’t accept a great deal of current at this low charging voltage; hence, the lower-than-expected charging rate of 30A.
A 90A car alternator can usually be expected to deliver around 45A continuously without overheating. As the 6mm wire is limiting the current to 30A, the alternator is quite happy.
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billskip
Well-Known Member
The words resistance and temperature remain constant....are you familiar with those?
If so why did you write this?....quote
"Ohms law is useful for circuit design to match components and voltages, but it doesn't affect power consumption which is P = I * V"
billskip
Well-Known Member
They are fitted, no need to add another....its called the accelerator pedal.
lustyd
Well-Known Member
Exactly, and the power available drops. No free lunch after all.
billskip
Well-Known Member
That must be a good tot, what is it ?
PaulRainbow
Well-Known Member
Still testing, but not getting as much charge from the alternators as i'd like, i think i'll get more from the DC-DC chargers.
geem
Well-Known Member
I agree. People forget that you can get half the alternator output continuously when using a DC/DC charger. Way more than you get from charging lead with an alternator
PaulRainbow
Well-Known Member
Still need to do more testing and experimenting with the charging, but with the charging going to the LA, which are in parallel with the Lithium, i'm seeing about 20A going into the Lithiums at the most. That's from a pair of 60A 24V alternators. I'll get 34A from the DC-DC chargers.
Interesting thing with loads, if i draw 150A from the Lithium bank there's an amp or so being drawn from the LA bank.
fredrussell
Well-Known Member
Just to answer a few people’s question - my alternator is definitely not a ‘smart’ one. The van is 20 years old (oh the shame!) - alternator replaced a few years back with the usual Bosch sort of thing.
William_H
Well-Known Member
Be careful when looking the battery as a resistance. Yes of course it has a resistance and more volts equals more current but a battery also has an opposing inherent voltage which opposes any charge. So it is the difference between the internal voltage (state of charge) versus the voltage applied to the terminals which might be better considered as relating to ohms law. However even that is fraught. ( actual internal resistance is very low)
So you apply a voltage to a battery (LA) and current flows according to the difference of charge state versus applied voltage. That current flow will develop a volt drop in the source and wiring resistance which will mean less voltage to charge at the terminals. The current then falls so less volt drop.
So we get a balance of current into battery that is not easily predicted by ohms law because the current accepted by the battery as related to voltage applied is very non linear. ol'will
William_H
Well-Known Member
An alternator will hopefully self regulate by dropping it's voltage when over loaded. The volt drop as explained above will reduce current to all services (by ohms law) but even more so to battery charge current.
Now a B2B charge has a different characteristic as with all switch mode power supplies it sucks more current if input voltage falls. (the regulator feedback circuits endeavour to supply what is wanted at output. So conceivably an alternator output might sag to 10v due to current over load but B2B will still try to pull lots of current. Potential for alternator damage.
Beware! (at least in theory)
On a different matter. If you have a LA battery bank in parallel with a Lithium bank then if LA is fully charged and lithium discharged you could in effect be trying to charge lithium from LA. With a huge current capability. We hope then that BMS will limit the current.
Now cheap lithium 50AH that I bought for mobility scooter specify a charge voltage of 16v and current limited to 40 amps. Does that imply that BMS will not necessarily limit charge current. or could the lithium batteyr be destroyed with too much current charging? (not much data vailable on BMS in the battery) ol'will
Now a B2B charge has a different characteristic as with all switch mode power supplies it sucks more current if input voltage falls. (the regulator feedback circuits endeavour to supply what is wanted at output. So conceivably an alternator output might sag to 10v due to current over load but B2B will still try to pull lots of current. Potential for alternator damage.
Beware! (at least in theory)
On a different matter. If you have a LA battery bank in parallel with a Lithium bank then if LA is fully charged and lithium discharged you could in effect be trying to charge lithium from LA. With a huge current capability. We hope then that BMS will limit the current.
Now cheap lithium 50AH that I bought for mobility scooter specify a charge voltage of 16v and current limited to 40 amps. Does that imply that BMS will not necessarily limit charge current. or could the lithium batteyr be destroyed with too much current charging? (not much data vailable on BMS in the battery) ol'will
geem
Well-Known Member
How long is the 6mm feed wire to the lithium?
fredrussell
Well-Known Member
Approximately 2 metres. Battery is under front passenger seat, conventional front-engined van.
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jakew009
Well-Known Member
There is a lot of crazy electrical theory in this thread 
BMS’s have absolutely no way of regulating charge current. They are a simple on / off device. Some use effectively a relay, others use fets. Either way, they are switched on until something bad happens (voltage too high, voltage too low, current too high) at which point they attempt to switch off and stop everything.
Drop in style batteries tend to have separate mosfets for charge and discharge, so they can disable charging without disabling discharge and vice versa.
As above, BMS is dumb. Cheap BMS may have smaller max current it can handle, but that just means once the current reaches its (firmware controlled) limit, the BMS will switch off and the current will go to zero.
Not unlikely, but impossible as above.
Not pedantic but utter nonsense. Current is equal in a loop no matter where you measure it.
Also absolute nonsense, max voltage is a USB cable is 48V. Maybe you have got confused between volts and watts.
This is bang on, and explains what is happening in OP’s post.
The 6mm wire is basically a massive resistor, and a 30W heater.
BMS’s have absolutely no way of regulating charge current. They are a simple on / off device. Some use effectively a relay, others use fets. Either way, they are switched on until something bad happens (voltage too high, voltage too low, current too high) at which point they attempt to switch off and stop everything.
Drop in style batteries tend to have separate mosfets for charge and discharge, so they can disable charging without disabling discharge and vice versa.
As above, BMS is dumb. Cheap BMS may have smaller max current it can handle, but that just means once the current reaches its (firmware controlled) limit, the BMS will switch off and the current will go to zero.
Not unlikely, but impossible as above.
Not pedantic but utter nonsense. Current is equal in a loop no matter where you measure it.
Also absolute nonsense, max voltage is a USB cable is 48V. Maybe you have got confused between volts and watts.
This is bang on, and explains what is happening in OP’s post.
The 6mm wire is basically a massive resistor, and a 30W heater.
jakew009
Well-Known Member
Correct, and this is basically why “drop in lithium” installed as per the OP is not recommended.
It’s relying on a set of circumstances (batteries at similar enough states of charge, wires introducing enough resistance, bad crimps introducing resistance) to make it behave in a way that you cannot necessarily control.
A future owner might replace the alternator, wonder why the fuse between the batteries keeps blowing, and decide to replace the fuse with a bigger one. The 6mm2 cable now melts and his van catches fire.
Hope being the key word. It will certainly not “limit” the current in the conventional sense of the word, but will hopefully sense the over current condition and immediately cut out “limiting” the current to zero.
It’s worth noting that there is no guarantee the BMS will cut out in an extreme over current condition. If the current that flows is big enough, the BMS won’t be able to interrupt it and the mosfets might fail closed. There is a warning in every BMS manufacturers data sheet about this and it’s the reason why high AIC fuses need to be fitted (such as Class T).
The BMS will definitely not limit the charge current (how on earth could it unless it had a way of changing the voltage??)
What it means is that you must use a charger that has a max current of 40A. In other words, the charger must regulate its voltage to ensure the current stays below 40A throughout the charge cycle.
If the charger just went straight in at 14.6V, and the battery was entirely flat (say 10V), hundreds/thousands of amps would flow and things would melt.
Instead the charger will start off at say 10.2V and gradually ramp up during the bulk stage as the batteries voltage rises keeping the current below 40A.
The reason why it needs to be limited to 40A is all batteries have a max charge rate. LFP is generally 1C max, so the max charge rate for a 50Ah battery is 50A.
Here is a random data sheet for a 50Ah cell, that shows the recommended and max charge rate (0.5C and 1C) on page 4:
https://energiepanda.com/wp-content...-50Ah-Rf-version-product-specification-en.pdf
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PaulRainbow
Well-Known Member
A fully charged LA battery is going to have a resting voltage of about 12.8V, a lithium battery will be at around that at 20% SOC, so unless the Lithium battery is totally flat, no current can flow. Plus, the BMS should stop the battery being totally flattened.
It isn't the purpose of the BMS to control charge current, only to stop it from exceeding it's set level. The BMS is there as a last ditch protection, usually against user mistakes. It's up to the installer to ensure the charging systems do not exceed the limits of the battery. Same applies with loads, temps, etc
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