Discharging lithium batteries to a too low voltage is bad and over-charging them is even worse. Like Mooly mentioned, lithium batteries may have slightly different rated voltages. Li-Pos for example usually have a nominal voltage of 3,7V and 4,2V when fully charged. But for example some smartphones use li-po/ion cells which go as high as 3,8V/4,35V.
In an application in which lifetime of the cells is more important than maximum performance, it's best to charge them to 4,1V only. Also, lithium batteries should not be depleted to less than ~20% of their capacity.
Lithium batteries should ideally be charged CC-CV (constant current & voltage). The charger also needs to cut off when the charging current is lower than about 1/10th of the initial charging current, otherwise the cell will keep drawing current and damage itself.
You definitely need to mod the O2's charging circuit. It needs atleast a cut-off at both high and low voltages. You also have to think about how the cells will be balanced when they're being charged. I generally recommend using cylindrical li-ion cells as they are tougher than "bag" type lipo's and much less spectacular in case of failure...[emoji1]
You could also take a look at BMS (battery management system) boards. There are some really small ones available, and they can take care of the cut-offs, current protection and even the charging!
In an application in which lifetime of the cells is more important than maximum performance, it's best to charge them to 4,1V only. Also, lithium batteries should not be depleted to less than ~20% of their capacity.
Lithium batteries should ideally be charged CC-CV (constant current & voltage). The charger also needs to cut off when the charging current is lower than about 1/10th of the initial charging current, otherwise the cell will keep drawing current and damage itself.
You definitely need to mod the O2's charging circuit. It needs atleast a cut-off at both high and low voltages. You also have to think about how the cells will be balanced when they're being charged. I generally recommend using cylindrical li-ion cells as they are tougher than "bag" type lipo's and much less spectacular in case of failure...[emoji1]
You could also take a look at BMS (battery management system) boards. There are some really small ones available, and they can take care of the cut-offs, current protection and even the charging!
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Yep, I screwed that one up!
That's what I get for posting late at night. The 3.7Vdc or so is the nominal (average working) voltage per cell, like UnknownAX said. The minimum voltage when the lithiums should cutoff will be somewhere around 3.1Vdc. The 3.1Vdc is for repetitive use of the battery to prevent long term damage. The absolute minimum voltage to prevent them from forming internal copper shorts and burning/exploding is around 2.7Vdc or so. For example here is a protection module made to attach to a cell. They have it set for a minimum of 2.4Vdc.
If you are using 2 lithium cells in series on each rail that would give a minimum of somewhere around 3.1Vdc x 2 = 6.2Vdc. You could try going back to NwAvGuy's original values of R25 = 2.74 meg and R9 = 40.2K, which gives a cutoff voltage of around 6.33Vdc from the bottom of that post I linked.
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Yeah I'm no expert on anything electronical so I'm going for just removing the batteries and recharge them with a dedicated charger. Instead of experimenting with any internal recharging. Maybe using the method of connecting the terminals of the charger to the amp like agdr described somewhere.
Yes I'm going to use cylindrical. For one thing because they seem to have significantly higher capacity for their volume than the "9v" lithiums. I also plan to use a thinner box for the amp.
I'm getting batteries with protection pcbs already equipped. Perhaps I'll look into the feat of integrating a battery management system board in the future. For now it seems easy enough to use an external charger.
Yep, I screwed that one up!
The 3.1Vdc is for repetitive use of the battery to prevent long term damage. The absolute minimum voltage to prevent them from forming internal copper shorts and burning/exploding is around 2.7Vdc or so. For example here is a protection module made to attach to a cell. They have it set for a minimum of 2.4Vdc.
If you are using 2 lithium cells in series on each rail that would give a minimum of somewhere around 3.1Vdc x 2 = 6.2Vdc. You could try going back to NwAvGuy's original values of R25 = 2.74 meg and R9 = 40.2K, which gives a cutoff voltage of around 6.33Vdc from the bottom of that post I linked.
Yes I think those values should be sufficient. Initially I was looking at discharge curves with pretty rapid discharging, but slower discharging means a little bit higher voltage in the usable stages. My concern was if the turn-on voltage of 6.95Vdc was to high for the latter part of the discharge-stage so the amp wouldn't turn on and so usable battery time would be wasted. But I think that 3.5V should be produced per cell at this rate of discharging. At the discharge-stages one would want to use anyway.
Btw what did you mean with
Would that be a problem here?
The O2 has a little problem there.
When the load on the NiMH batteries is cut off by the O2's power management circuit when the batteries are low, the battery voltage tends to rise back up a bit. If it rises too much it passes the O2 power management circuit's "on" trip point and turns the O2 back on again. That re-loads the batteries causing the battery voltage to drop again, causing the PM circuit to quickly go past the "off" trip point and shut the thing down again. And over and over. It will just sit and oscillate like that and make a "motorboating" sound in the headphones as the power to the chips is turned on and off at a rapid rate by the PM circuit.Those PM values NwAvGuy had in the original V1.0 of the O2 did that rather badly, the R25=2.74 meg and R9=40.2K. He then made a stab at fixing the problem a week or two after the O2 was announced by changing R25 to 1.5 meg and R9 to 33K. Notice how that spread between the "off" voltage and "on" voltage changes for the different resistors values in the post I linked (hysteresis). That works a lot better, but the problem still remains for some folks. From personal experience I've found the batteries do that voltage "rise up when load is removed" bit more when they are old. I always tell folks who have the motorboating problem to try new batteries first.
I designed a power management "latch" circuit too, this small PC board which plugs in place of the O2 NJM2903 comparator chip. It "firmly" latches the O2's power management circuit "off" when the PM circuit trips the first time. Powering the O2 off for 15 seconds (after the batteries are recharged if it turned off due to low batteries or run on AC) resets the latch circuit. The schematic is at that link along with the part values. The circuit works great, with the exception of an unintended "feature" that is annoying and I haven't had time to figure out a solution for. If the O2 is running on batteries and the AC power is plugged in it triggers the latch circuit.
The O2 has to be power-reset for 15 seconds to turn it back on.So long story short, I really don't know if lithium cells have that same behavior of the voltage rising up when their load is cut off as the "9V" NiMH do, and if so how much it rises. Depending on that you may or may not have that same "motorboating" problem with the O2 PM circuit with his original 2.74M and 40.2K values. If you do, consider giving that PM latch circuit a try if you can live with that "latch on AC connect" issue. You may also be able to just spend some time with the math I have in that post and find some combination of R25, R9, and R5 values that would preserve the 6.2V "off" trip point but raise the "on" trip point to a higher voltage (more hysteresis in the circuit). Mooly had a suggestion several posts back for a change in one of those resistors to increase the hysteresis.
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So I fried my power section (don't ask) but everything still works via battery.
I popped on back here to see what I can do and after reading the last few pages of posts about using li-ion I went for it!
Maybe this will give some ideas
Connector is JST-PH
http://pages.ebay.com/link/?nav=item.view&alt=web&id=230779390299
Battery is from my Blade 130x
2s1p 300mah pack
http://hurricanemodels.co.uk/engine...2s35/E-Flite+300mAh+2S+7.4V+35C+Li-Po+Battery
I pulled the middle pin out of the connector and the connector fits perfectly into the holes!
Only one out of the three posts for each battery terminal connects to the circuit and the two closer ones which the JST connector reaches are isolated so I had to jumper then to the right junction.
Overall a pretty clean setup!
Only problem is, you have to take batteries out to charge them
Sent from my iPhone using Tapatalk
I popped on back here to see what I can do and after reading the last few pages of posts about using li-ion I went for it!
Maybe this will give some ideas
Connector is JST-PH
http://pages.ebay.com/link/?nav=item.view&alt=web&id=230779390299
Battery is from my Blade 130x
2s1p 300mah pack
http://hurricanemodels.co.uk/engine...2s35/E-Flite+300mAh+2S+7.4V+35C+Li-Po+Battery
I pulled the middle pin out of the connector and the connector fits perfectly into the holes!
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Only one out of the three posts for each battery terminal connects to the circuit and the two closer ones which the JST connector reaches are isolated so I had to jumper then to the right junction.
Overall a pretty clean setup!
Only problem is, you have to take batteries out to charge them
Sent from my iPhone using Tapatalk
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So I fried my power section (don't ask) but everything still works via battery.
I popped on back here to see what I can do and after reading the last few pages of posts about using li-ion I went for it!
Maybe this will give some ideas
Connector is JST-PH
Battery is from my Blade 130x
2s1p 300mah pack
I pulled the middle pin out of the connector and the connector fits perfectly into the holes!
Only one out of the three posts for each battery terminal connects to the circuit and the two closer ones which the JST connector reaches are isolated so I had to jumper then to the right junction.
Overall a pretty clean setup!
Only problem is, you have to take batteries out to charge them
Nice! So how's it performing compared to when using the external power input?
Works great! Can't really tell much of a difference. I haven't ran through a cycle of battery life yet. Interested to see at what voltage it will start motorboating.
Sent from my iPhone using Tapatalk
I started to notice a crackling sound through my headphones (tried different headphones) that lasts about 1-2 seconds and happens at random times. Once a week or a few times a hour so its hard to pinpoint if it really is the amp. Other than that, there is no noticeable audio problems. Has anyone come across this problem on their o2? I'm also using a Fiio DAC and my computers SPDIF output.
Right channel only.
Volume does not affect it, I could have it at max or very low. The crackling sound is always the same volume which is very low. At first I thought it was the ear pads rubbing against my beard but it also happens with different types of headphones.
I will open it up today and recheck the joints but I'm fairly certain everything is ok as I double checked everything when I built it.