Mach2 Slew Speed, Power, and Voltage Question #Mach2GTO


michaeljhanson@...
 

Hi Greg,

I hear two different questions.
1) What is the max permissible input voltage to the CP5?
2) Can I use a 24V battery charger.

For question 1, no one likes to hear, "it depends".  So, the keypad has a max input voltage of 28.0V.  This is where the 28V "specsmanship" number, mentioned in the other thread, comes from. 

This answer is unsatisfying, so we can be more pragmatic by entertaining question 2 instead of question 1.  I think this is really what you are wanting to understand.
A proper three-stage charger can provide up to 29.4V absorption voltage at the max absorption set point.  This exceeds the keypad rating.  Are you even using a keypad???
If not, the next "limit" is the transient suppression, which can begin conducting at 29.5V.  While the margin sounds low, the suppression voltage has a large positive temp coefficient when it begins to conduct, and, there will be losses in the power cable to the CP5 contributing to margin.  So, the margin is much higher than it seems.  So, if you do not have a keypad plugged in, I see no issue using a battery charger. 

Suppose you DO have a keypad.  Do you know the absorption set point of the charger?  Does the charger even have an absorption set point? If so, do you have control over it?  Does it transition from absorption to float properly (does it even have a float stage)?   Can you measure the highest charge voltage?  If the answers are "no", I would be reluctant to have a keypad plugged in while charging.  Reason is, there's just too many crappy chargers out there, with no possibility of us testing every one of them.  An absorption voltage set point can be as low as 28.2V.  We know that, even with the mount parked, there will be a couple tenths drop in the power to the keypad through two harnesses.  *IF* you know the absorption voltage to be at the low end of typical, I see no issue using a charger even with a keypad.

I hope this helps.

Regards,
Mike Hanson


Greg McCall
 

Hi Mike,
The first two things are that I don't use a keypad and I don't use or need a 3 stage charger. But I was also not just trying to solve my particular issue but to provide info so anyone who needs it is armed with the information. So an answer specific to me while very helpful, would be better for all if I could get AP to document the area.

While I've seen many discussions of the power bricks, I only do portable Astrophotography as I live in the centre of Sydney and must drive to a location. I currently use 12v (nominal) LiFePO4 batteries but was considering a 24v battery to power my Mach2. The alternative, a 12v to 24v booster (readily available from China) could be used, but I wonder about the quality of those available and the noise produced on the 24v output. This also seemed an obvious solution and wonder why AP has not suggested it leading me to think it may not be a good idea.

Lead Acid batteries, typically AGM or Gel construction for astro stuff, need a 3 stage charger with the absorption voltage at about 80% charge level which is around 40% of the charging time.
I use Lithium Iron Phosphate batteries (LiFePO4) which are a type of lithium that are close to matching Lead Acid voltages. 
These are a safe type of Lithium battery (compared to Lithium Cobalt-based batteries is used in say a mobile phone) and also used in Recreation Vehicles, camping etc. They also charge at full charge current for 100% of the time, can be safely discharged to around 80% compared to 50% of lead-acid, are under 1/2 the weight and have way more charge-discharge cycles.
I particularly favour some locally made (in Australia) high-end batteries (read trusted brands) that have a built-in state of charge and current monitoring as part of the internal battery monitoring system, that I can BlueTooth too with an app if needed during the night.

LiFePO4 batteries have some slightly higher voltages compared to Lead Acid.

The reason I would have liked to see a definitive specification is that I know what gear I have and AP know what they have. I then can choose a solution that meets both specifications.
I didn't expect AP to test the variations (particularly on a world stage). Specs take the guesswork out of decisions and not rely on what the world seems to be moving to opinion-based decisions based on numbers of votes. Many opinions actually don't override facts and I would have liked to see the facts from AP in the form of a spec or definitive recommendation.

I also understand voltage drop and the need to err for thicker cable with shorter runs. I also have multimeters to confirm the various voltages. I typically use oversized cables, quality fuses and powerpole connectors to reduce the voltage drop.

I also only use quality chargers. In my case, I favour Victron smart chargers with the Bluetooth app.
The charger also has its own cabling and fuse to the battery.

regards
Greg


John Upton
 

Greg,

On Thu, Apr 8, 2021 at 04:40 PM, Greg McCall wrote:
While I've seen many discussions of the power bricks, I only do portable Astrophotography as I live in the centre of Sydney and must drive to a location. I currently use 12v (nominal) LiFePO4 batteries but was considering a 24v battery to power my Mach2. The alternative, a 12v to 24v booster (readily available from China) could be used, but I wonder about the quality of those available and the noise produced on the 24v output. This also seemed an obvious solution and wonder why AP has not suggested it leading me to think it may not be a good idea.
   One thing to keep in mind is that using a 12v to 24v DC-to-DC converter is that you might still have the same issue with running a charger during operation.

   My own LiFePO4 battery has a charge profile that requires up to 14.4 volts during the final absorption / equalization phase of the charge cycle. (The charger should then immediately switch to either float or monitoring mode.) The problem I would have is that the 12v to 24v power converter has a maximum input of voltage specified at 14.0v. Thus, charging the battery with the power converter attached could damage the converter unit.

   My own solution to this is to use my charger in "Supply Mode" where is becomes a fixed voltage float charger or "power supply" outputting a fixed regulated 13.6 volts. This allows some recharging of the battery while in operation without endangering either the battery or the voltage converter. Many of the high-end chargers have a "Supply Mode" setting -- both my CTEK and NOCO chargers support that manual form of operation.

   If you wish to use a normal Lithium charger on your battery during operation, you will need to carefully scan the specs for whichever voltage converter you use to ensure it remains in spec during a charge cycle of your particular charger.

   Regarding the output of the voltage converters, I found the one I use to be pretty good. (I am not taxing it at all. It is spec'ed for 24v @10A output and the highest consumption I have measured on the 12v side is about 6.5 to 8.5 Amps.) I did monitor the output both steady state and during a voltage transient on the input side. At steady state there is about 0.2v to 0.4v ripple on both input and output due to the five switching mode voltage converters (5v, 12.6v, 13.7v, 19v, 24v) contained in my field battery box. The voltage transient happens when I power up my NUC using another 12v to 19v converter as indicated in my second message in this thread. Due to the impedance of the wiring in my battery box wiring, I see a ~75 uSec drop to 10v on the 12v input side. (That is the cause of the motor stall issue I mentioned in my second post in this thread. This takes about 3 mSec to fully recover back to 13.3v. The output of the 24v lead to the Mach2 shows only a similarly short drop of about 1v and fully recovers within 150 uSec as seen on the scope trace below.



   In the trace above, the baseline 0v level is the bottom grid line. Each grid is 5v vertically and 100 uSec horizontally.

John


Roland Christen
 

I have a trickle charger on mine and as long as the CP4 is drawing power, the voltage does not rise much above 24 volts. Perhaps a high current charger will produce a higher voltage because the mount is not using that much current. I also am monitoring the voltage and power with an in-line meter while the mount is running.

Rolando


-----Original Message-----
From: Greg McCall <emailgregnow@...>
To: main@ap-gto.groups.io
Sent: Wed, Apr 7, 2021 3:55 pm
Subject: Re: [ap-gto] Mach2 Slew Speed, Power, and Voltage Question #Mach2GTO

Rolando,
You don't charge a 24v battery at 24v.
Just checking one battery spec at https://enerdrive.com.au/product/epower-b-tec-24v-100ah-lithium-battery/
It can be 28.8v-29.2v for charging and 27-27.6v as a float voltage 
(that's for a lithium battery, specifically, LiFePO4 often used in astro setups, camping, RV industry)
24v is a nominal voltage with the operating range being 22v to 29v 
While I've not been given a specification for the allowed voltages, the discussion so far has indicated these voltages are not suitable for the Mach2

Contrary to what has been said in this message thread (and a similar thread), are saying I can use a 24v battery on the Mach2 while charging?
I just see no specification and now mixed messages.

--
Roland Christen
Astro-Physics


Greg McCall
 

Hi John,
That's excellent information and I much appreciate the detail. You have also raised something that I didn't consider yet with those popular power supply modules as I had not really seriously considered a 12v to 24v converter. You have also saved some effort in those current measurements so again, thanks for that detail.

(I wonder what the 12v current is just operating the Mach2 at 12v.  Something to watch next time. -- The AP spec of 5A minimum is not very helpful as you usually plan for a maximum both in power supply, cabling & fusing)

My LiFePO4 batteries use 14.6v but I know I can set the max on my chargers if needed and the float voltage is 13.8.

I also use a NUC with a 12v to 19v converter. It's designed to power a laptop in a car so I assumed it could handle at least the charge voltages of lead-acid. (just replaced the cigarette lighter plug with a powerpole connector)

cheers
Greg