anguslau

Just want to chime in here since power is one of my biggest headache for traveling as well.

During my recent trip to Australia, I ended up buying a lead-acid battery locally. Apart from wasting the battery afterwards, its also quite expansive and troublesome to arrange.

This time, I'll attempt to use LiPo batteries as advised by some forum members. Hope it will work fine...

Here are the details of my case for your reference. Experienced users please correct me if I've done something inappropriate :

1) I bought three 11.1V 4.5AH batteries in total from model shops near Kwong Wah Hospital. They are very expensive ($500 each), but light (~350g each) and compact. I heard that there are limitations in carrying these aboard airplanes (~2 packs). So I might need help from others to share the load if they really checked.

2) Bought a balanced charger (forgot price, several hundred). It is also quite light (~700g) and compact. It can fully recharge a 4.5AH battery in <2Hrs if I set charging current to 3A. This is much faster than lead-acid. So I can charge batteries during the night if close to AC power supply.

3) LiPo batteries have nominal voltage of 11.1V, but can vary from ~9V to 12.5V when operating. I worry that some equipment may have problem with the lower voltage range, particularly CCD cameras. So I bought a voltage regulator which takes 8-36V and outputs 12V with ~85% efficiency (claimed). The regulator weights about 450g and is not too bulky.

4) I have also built a cable harness which allows me to drive up to three loads from the regulated output and connect up to two batteries on the input side. I can connect a voltmeter to monitor the battery voltage online. When it drops to about 10V, I can hot swap batteries without power down the equipment. I read that LiPo batteries should not be run below 9V, but there is no way to tell unless you monitor it. The hot swap action may be a potential problem since I am connecting a fresh battery with a discharged one in parallel momentarily. Large current may flow within the two batteries. I measured the internal impedance of the battery to be about 0.25 Ohm with a current flow of about 4A for a voltage difference (between the two batteries) ~1V. For 2V difference, 8A may flow which should be within safe zones for the battery since they allow quite large currents. I have checked in practise, the battery did not get warm.

The total setup is about 2.2Kg, gives ~13.5AH standby power with potential recharging when AC supply is nearby. It may be an acceptable travelling solution if it proves to work fine during this trip. Attached is a sketch of the above system and a discharge curve of a LiPo cell for your reference. Knowledgable forum users please help me to correct any problems or improve my setup. Many thanks!

sn-10

To your point 4, swapping a fully charged LIPO to your parallel circuit is not recommended because both the used and the newly replaced ones are being drawn current simultaneously, eventually the used one will be over-discharged. I would suggest separate the mount and your CCD powers by using a smaller one, say 2200mah, only for the mount; it is sufficient for 5-6 hrs. Or even use the same 4500mah that will last 12 hrs.

These batteries are designed to be capable of giving hundreds of Amp sustaining for seconds (it gets damn hot after that!), your application surely will not raise the temperature significantly.

To #3, the discharge curve is for 6C discharge rate, in your application you are discharging at 0.5C max therefore the curve will be much flatter, voltage drop may not be noticable until closing to depletion.

To #2, usually we can charge a lipo at 1C. Your battery can be charged at 4.5A theorectically, BUT in reality even if you set the charging rate at 1C, it charges at such rate only at the very beginning. The charger screens the voltage over time and automatically adjust the charging current which will eventually drops to as small as 0.1A, that is called a "drip-charging" stage when the battery is close to fully charged. Just to clear some technical issue, and yes, your battery can be charged in 2 hrs.

[ 本帖最後由 sn-10 於 2009-10-16 12:21 編輯 ]

anguslau

Thanks very much for advises and info, SN-10!

The amount of time the old and new batteries are in parallel will only last for several seconds during the hot swaps. I understand it is not a good idea to join them together for any sustained period of time. My idea was that as one battery becomes discharged (to ~10V), I'll connect a fresh battery in parallel. Then disconnect the discharged battery immediately. The main advantage for this "hot swap" is that I don't need to power down any equipment, particularly if say the CCD camera is in the middle of a long exposure. I am hoping the voltage spikes during a hot swap can be absorbed and shielded by the voltage regulator.

Let everything run off the voltage regulator (in stead of separating the batteries for different equipment) ensures that they all get 12V nominal. Further more, I can monitor the battery voltage continuously, avoiding any of the batteries running at too low a voltage (ie <9V). Anyway, this is my wish...

[ 本帖最後由 anguslau 於 2009-10-16 13:27 編輯 ]

sn-10

I see, I thought you are running them in parallel all time.

BTW, a cell should be kept at 3.7V (below that is considered overdischarged), thus for a 3s the voltage should be 11.1v, not 9v. A fully charged one cell is 4.2v, the overall V max is 12.8v.

Raymond_TSE

I went to Red Earth at Yunan province 3 years ago arranged by Wong Sir and Kuming So.

Due to the cold weather at the end of December and 2.5km uphill (near 0C at night), I decided to bring a 60 feet cable to drag
AC electricity from their very primitive guest/canteen wood hut to the open area where I placed the tripod.

The power adapter for my modi 350D, autoguider and notebook worked all night long without problem for 2 nights, even though the
notebook screen was frosted with ice. I should have brought a heater to warm up myself.

So, unless you are in an open carpark or open ground far away without any nearby guesthouse, the best solution is
AC power, not dry batteries.

That is the reason why I suggest a briefing meeting for the imagers to solve out various problems and to co-ordinate
efforts as to who to bring what to address these problems.

We may end up with our imagers seeing each other at the airport bringing rougly similar and repetitous equipments.
The fixed tripod imagers has less to worry about, however.

Raymond Tse

Mr.Big

打風:u_21 :u_21 :u_21

tang

風來風去,10月中旬大陸北部多有高壓脊南下,
把風壓向海南島方向,何須擔心影響台灣！

anguslau

I will attend the pre-trip briefing if there is one. Other than Raymond, anyone else interested in meeting before hand?

原帖由 Raymond_TSE 於 2009-10-16 16:07 發表
I went to Red Earth at Yunan province 3 years ago arranged by Wong Sir and Kuming So.

Due to the cold weather at the end of December and 2.5km uphill (near 0C at night), I decided to bring a 60 feet  ...

anguslau

原帖由 sn-10 於 2009-10-16 13:41 發表
I see, I thought you are running them in parallel all time.

BTW, a cell should be kept at 3.7V (below that is considered overdischarged), thus for a 3s the voltage should be 11.1v, not 9v. A fully ch ...

Thanks for the reminder, sn-10. I am a bit confused on the safe low voltage though. Looking at the 6C discharge curve, the cell voltage drops below 3.7V very quickly. Draining the battery at a slower rate may change the shape a bit, but it appears that the battery still have much power left at 3.7V. Setting the low cell voltage to 3.5V seems to be more appropriate?? This will translate to a 3S battery of 10.5V?

Subaru

Angus,

I bought 2 monitoring devices for LiPO usage in the field, one is an alarm for 3S and the other is a portable meter for up to 6S.  I use the meter in the field work in HK, and it is fine.

You may consider these portable devices instead of a bigger size multimeter.

tang

各位朋友,剛收到林啓生先生電郵,23日會到台中機塲接機,他有以下幾點提出, 請大家商討:

1. 祇能提供一顆50AH 12V及一顆7AH 12V的電瓶 , 2~3條電線(可由大電池分接到幾部赤道儀用)以及2顆5kg高橋用平衡重鎚 .
2. 是否需要借用他的車子,在山上提供接送,(我認為如果有人當司機,可以向他借用,方便日間接載大家四處遊覽,萬一清境不理想,
    可以揸往合歡山影星,當然要付回油錢)
3. 他提議大車先到台中高鐵站接6位朋友,然後才到機塲接其他朋友,時間比較配合

以上各點,請大家比意見